There's been much debate as to how to properly install RS side skirts onto a non-RS Impreza. After meticulously going through the process myself, I figured I'd save everyone else the headache of trying to figure it out on their own. Bear in mind I did this on a 1998 L sedan, so there may be slight variations for other years/trim packages.
First, here are the parts you'll need:
Using your Phillips screwdriver, remove the screw from the black plastic square grommet on the underside of the rear of the rocker panel (right in front of the rear wheel). Attach the rear side skirt piece by inserting a metal screw into the black grommet (move the black rubber piece to the back of the black grommet). Adjust the side skirt piece to where you like it, and mark its location on the rocker with pieces of masking tape (just mark off the perimeter of the side skirt piece so you know where it fits when the piece is off). Now, remove the side skirt piece.
Side Clips
With the rear side skirt piece removed, insert the gray plastic male clips that attach the side skirt to the side of the rocker. With your tape measure, measure from the inside edge of the rear of the side skirt to the center of the first plastic clip. Using your awl, scratch a vertical mark at the same distance from the rear edge of the rocker on the car. Now, measure between clips (using the same point on each clip - center, right edge, or left edge) and make the corresponding marks on your car. There are only three clips on the rear piece so this shouldn't take too long.
Now that you've got your horizontal distances calculated, it's time to determine the vertical distance, which can be a bit trickier. Really, because the side skirts are plastic and consequently flexible, so long as all the marks are all on a straight line, it should look fine. I found that when Subaru sprayed the textured coat of protectant onto my Impreza, it left a perfect guide for figuring the vertical distance for the side skirt clips. If your car has had the underbody texture sprayed, look closely along the rocker panel and you should see a straight line where the rocker was masked off when the texture spray was applied - this is your guide. If you don't have this incidental guide, just make a line approximately 1" from the bottom of the door all along the rocker panel. Where this line (either the one from the texture or the one you measured) intersects the vertical marks you scored earlier, is where you want to drill.
Consistent with the adage, "Measure twice, cut once," hold the side skirt piece (with the gray clips still inserted) up to the rocker panel to make sure your marked points are accurate.
To ensure your pilot bit does not wander, use your awl and hammer to tap a small dent where your vertical and horizontal marks intersect. Using your drill with the 1/16" bit, drill pilot holes at each mark. Once pilots are drilled, again, hold the side skirt piece (with the gray clips still inserted) up to the rocker panel to make sure your pilot holes are accurate.
Pilot hole drilled - if you look close, you can see the line made by the textured protective coating.
If everything lines up alright, use your stepped bit to drill the pilot holes to 5/16".
If you live in a rust-friendly area, now's the time to apply corrosion protection to your newly-drilled holes.
Insert the black plastic female clips.
Now, attach the rear side skirt piece.
With the rear skirt piece on, remove the black plastic rivet/clip from the inside of the front wheel well and attach the front side skirt piece (similar to how you did the rear piece) using the same plastic clip. You know the drill, hold the skirt into place, and mark it off with masking tape.
Repeat the same process as the rear: remove front skirt piece, attach gray clips, measure from inside of front skirt to first clip, mark corresponding point on rocker, measure between clips, mark points, etc. You should still have your horizontal line marked (if you don't, you'll have to do it again), so once you double-check your drill points by holding up the skirt with clips for comparison, you should be ready to awl punch, pilot drill, and 5/16" drill.
I cannot stress enough the importance of making certain that your scored marks and then pilot holes are accurate. Once that 5/16" hole is drilled, there's no going back.
Attach the front skirt piece using the clips and then go take a break.
Under Screws
This is the easy part. Now that your skirts are semi-attached, simply poke your head under the rocker and, using your hammer and awl, mark the points the screws should go by tapping the awl into the rocker. Try to center the marks to the holes in the skirt pieces. Once all the marks are made, pull the skirts back off.
Same as before, use the 1/16" bit to drill pilot holes.
Now, use the stepped drill bit to drill 3/8" holes.
When you're done drilling, you should have five, neatly-drilled 3/8" holes in the underside of your rocker.
And now the fun part.
It's not often that you need to square holes. I was told many times to forget about square holes and just use round grommets with clips or self-tapping screws. I was set on using the OEM hardware, so I asked my dad, who has a machine shop where he works, if he could make me a beveled square punch. Ask and ye shall receive...
Hardened Steel + Precision Machining = Square Holes
Chances are, you can buy (or make) something comparable. If you absolutely cannot, I *might* be able to convince my dad to make another punch, at a cost, of course.
With your punch, insert it into the round 3/8" holes and begin tapping with your hammer. The holes toward the front only go through one sheet of steel, so it's pretty light - towards the center of the rocker, you have to go through two sheets, so it can take a bit of elbow grease.
Just keep tapping away, occasionally checking the white plastic square grommet for fit (you want to be able to insert the grommet most of the way by hand, but need the hammer to tap it all the way in). Your square hole should be to-size when the punch has eliminated most (if not all) of the round shape from the 3/8" bit.
Again, the fit of the grommet should be very snug.
Now, all the female hardware pieces should be attached, and you're minutes away from side skirt goodness.
Attaching the Side Skirts
Before you just slap the side skirts back on, you should attach the protective black rubber strips. I've been asked whether these are really necessary, and the answer is: probably. You don't actually "need" the protective strips to attach side skirts, but it would be invariably foolish to not install them, as they keep dirt and rain out from getting underneath the side skirt (and they prevent the skirts from messing up your paint).
The protective strips utilize an adhesive. Make sure you wipe the top inside lip of each skirt piece, remove the brown paper backing on the strip, and carefully attach it to the skirt piece, making sure to press the strip firmly once it's in place.
You'll find there's a bit extra on each piece, so grab your razor blade and trim off the excess.
Now that the skirts are ready to be attached, start with the rear piece and pop it in, using the the side clips. With your 10 mm socket and wrench, insert the screws into the white grommets underneath. These should be snug, but not overly-tight (remember, you're screwing into plastic, not steel). Make sure the rubber protector strips don't get folded under while attaching the skirts. Repeat the process for the front skirt, and re-insert the black rivet/clip into the front wheel well.
Ta-da! One side down, one side to go. Obviously, all you have to do is repeat the entire process for the other side.
If you have any questions, just ask. Thanks for reading!
First, here are the parts you'll need:
Once you have all the parts, here are the tools you'll need:
First, jack up one side of the car. Make sure it's high enough that you can drill underneath the rocker. Once the car's up, get a rag and some all-purpose cleaner and wipe down the rockers really well - you want them to be free of dirt and grime.
Using your Phillips screwdriver, remove the screw from the black plastic square grommet on the underside of the rear of the rocker panel (right in front of the rear wheel). Attach the rear side skirt piece by inserting a metal screw into the black grommet (move the black rubber piece to the back of the black grommet). Adjust the side skirt piece to where you like it, and mark its location on the rocker with pieces of masking tape (just mark off the perimeter of the side skirt piece so you know where it fits when the piece is off). Now, remove the side skirt piece.
Side Clips
With the rear side skirt piece removed, insert the gray plastic male clips that attach the side skirt to the side of the rocker. With your tape measure, measure from the inside edge of the rear of the side skirt to the center of the first plastic clip. Using your awl, scratch a vertical mark at the same distance from the rear edge of the rocker on the car. Now, measure between clips (using the same point on each clip - center, right edge, or left edge) and make the corresponding marks on your car. There are only three clips on the rear piece so this shouldn't take too long.
Now that you've got your horizontal distances calculated, it's time to determine the vertical distance, which can be a bit trickier. Really, because the side skirts are plastic and consequently flexible, so long as all the marks are all on a straight line, it should look fine. I found that when Subaru sprayed the textured coat of protectant onto my Impreza, it left a perfect guide for figuring the vertical distance for the side skirt clips. If your car has had the underbody texture sprayed, look closely along the rocker panel and you should see a straight line where the rocker was masked off when the texture spray was applied - this is your guide. If you don't have this incidental guide, just make a line approximately 1" from the bottom of the door all along the rocker panel. Where this line (either the one from the texture or the one you measured) intersects the vertical marks you scored earlier, is where you want to drill.
Consistent with the adage, "Measure twice, cut once," hold the side skirt piece (with the gray clips still inserted) up to the rocker panel to make sure your marked points are accurate.
To ensure your pilot bit does not wander, use your awl and hammer to tap a small dent where your vertical and horizontal marks intersect. Using your drill with the 1/16" bit, drill pilot holes at each mark. Once pilots are drilled, again, hold the side skirt piece (with the gray clips still inserted) up to the rocker panel to make sure your pilot holes are accurate.

Pilot hole drilled - if you look close, you can see the line made by the textured protective coating.
If everything lines up alright, use your stepped bit to drill the pilot holes to 5/16".

If you live in a rust-friendly area, now's the time to apply corrosion protection to your newly-drilled holes.
Insert the black plastic female clips.

Now, attach the rear side skirt piece.
With the rear skirt piece on, remove the black plastic rivet/clip from the inside of the front wheel well and attach the front side skirt piece (similar to how you did the rear piece) using the same plastic clip. You know the drill, hold the skirt into place, and mark it off with masking tape.
Repeat the same process as the rear: remove front skirt piece, attach gray clips, measure from inside of front skirt to first clip, mark corresponding point on rocker, measure between clips, mark points, etc. You should still have your horizontal line marked (if you don't, you'll have to do it again), so once you double-check your drill points by holding up the skirt with clips for comparison, you should be ready to awl punch, pilot drill, and 5/16" drill.
I cannot stress enough the importance of making certain that your scored marks and then pilot holes are accurate. Once that 5/16" hole is drilled, there's no going back.
Attach the front skirt piece using the clips and then go take a break.
Under Screws
This is the easy part. Now that your skirts are semi-attached, simply poke your head under the rocker and, using your hammer and awl, mark the points the screws should go by tapping the awl into the rocker. Try to center the marks to the holes in the skirt pieces. Once all the marks are made, pull the skirts back off.

Same as before, use the 1/16" bit to drill pilot holes.

Now, use the stepped drill bit to drill 3/8" holes.

When you're done drilling, you should have five, neatly-drilled 3/8" holes in the underside of your rocker.

And now the fun part.
It's not often that you need to square holes. I was told many times to forget about square holes and just use round grommets with clips or self-tapping screws. I was set on using the OEM hardware, so I asked my dad, who has a machine shop where he works, if he could make me a beveled square punch. Ask and ye shall receive...

Hardened Steel + Precision Machining = Square Holes
Chances are, you can buy (or make) something comparable. If you absolutely cannot, I *might* be able to convince my dad to make another punch, at a cost, of course.
With your punch, insert it into the round 3/8" holes and begin tapping with your hammer. The holes toward the front only go through one sheet of steel, so it's pretty light - towards the center of the rocker, you have to go through two sheets, so it can take a bit of elbow grease.

Just keep tapping away, occasionally checking the white plastic square grommet for fit (you want to be able to insert the grommet most of the way by hand, but need the hammer to tap it all the way in). Your square hole should be to-size when the punch has eliminated most (if not all) of the round shape from the 3/8" bit.

Again, the fit of the grommet should be very snug.

Now, all the female hardware pieces should be attached, and you're minutes away from side skirt goodness.
Attaching the Side Skirts
Before you just slap the side skirts back on, you should attach the protective black rubber strips. I've been asked whether these are really necessary, and the answer is: probably. You don't actually "need" the protective strips to attach side skirts, but it would be invariably foolish to not install them, as they keep dirt and rain out from getting underneath the side skirt (and they prevent the skirts from messing up your paint).
The protective strips utilize an adhesive. Make sure you wipe the top inside lip of each skirt piece, remove the brown paper backing on the strip, and carefully attach it to the skirt piece, making sure to press the strip firmly once it's in place.

You'll find there's a bit extra on each piece, so grab your razor blade and trim off the excess.

Now that the skirts are ready to be attached, start with the rear piece and pop it in, using the the side clips. With your 10 mm socket and wrench, insert the screws into the white grommets underneath. These should be snug, but not overly-tight (remember, you're screwing into plastic, not steel). Make sure the rubber protector strips don't get folded under while attaching the skirts. Repeat the process for the front skirt, and re-insert the black rivet/clip into the front wheel well.

Ta-da! One side down, one side to go. Obviously, all you have to do is repeat the entire process for the other side.
If you have any questions, just ask. Thanks for reading!