Join Date: Sep 2012
Location: Wilkes-Barre, PA
Hammer's spot on. I find it easier to trim a window when the jamb is slightly inset from the drywall, rather than when the jamb is sticking out. I tap the edge of the wallboard with a hammer to crush it, stopping about 2/3 the width of the casing back.
A little trick for cutting your miters in this situation that Hammer also touched on: for a back-cut 45, set your saw to about 44.5 degrees. Position the casing with the outside edge against the fence and lift the edge closest to you off the saw table slightly - about 1/16". (You can use a shim to steady it when making the cut if you prefer.) This produces a very slight compound with a miter of 45 and a back bevel of roughly 1 degree which hollows out the back of your pieces enough to ensure that the profile faces have no gaps. You may have to play with it a bit until you get the hang of it.