It's right under the water spout, located on the bottom of the driver's side on the front of the motor. Just follow your lower radiator hose where it goes into the motor. 2 bolts hold on the water spout and the t-stat and gasket should pop right out.
There's probably no real DIY because it's very straight forward

 

There's a thick black hose that goes to the bottom of the engine on the driver's side from the bottom of the radiator. There should be two 10mm (I think) bolts holding the fitting to the bottom of the engine. Take those two out, pop the hose off the bottom, pop the thermostat out and replace in reverse order.

 

It's so easy, almost easier than an oil change.

If you search and dont find something here, then google is your friend.

http://lmgtfy.com/?q=how+to+change+subaru+thermostat

 

Just keep your head back when you pull it loose, lest you get assaulted with a gallon of coolant.

 

And be very careful tightening those bolts back down, you're going into aluminum and the water neck doesn't need to be on *that* tight. Also, take note on how the gasket seats on the thermostat itself. I managed to have the gasket pop off before I saw how it went on and I was trying to sandwich it between the thermostat and the block like a dummy before I realized it goes around the edges of the thermostat itself.

 

thank you all for the quick responses. looks like this will be the next part of my pre-winter maintenance along with a coolant flush.

 

Thanks everyone. This was a big help

 

Thanks everyone. Big help

 

well changed mine today while doing a coolant flush. but when i say change im talkin about just the gasket. it was covered in scale, but the thermostat itself was in great condition. the new one i bought (stant) was about half the size (springwise) and didnt have a hole for a bypass. so i just reused the same thermostat with a fresh new gasket. better quality won.

 

Isnt it so easy? I did mine in 20 degree weather last winter and hated my life

 

it was pretty easy. good location. one bolt was rusted and rounded on me but the next size smaller socket got that lil bugger out no problem.