I didn't find any DIYs specifically on spark plugs, but here's a link to the service and shop manuals.
http://www.ft86club.com/forums/showthread.php?t=8448
Having replaced spark plugs on other Subarus, it's not difficult, it's just time consuming. You'll need a few extensions and universal joints to get in there, and don't let yourself get frustrated when things are in your way, just figure out how to move them or work around them.
Edit: Reading other forums, apparently you need to lift the engine slightly to get to the rear spark plugs.