I have to agree that you may be better off (and you will be cool sooner) if you get a pro.
The boys on the board may make fun of me here...
I'm embarrassed to admit it, but the first time I installed a through the wall AC, I framed up the hole a little too tight. Months later after the drain hole in the AC became clogged and the water leaked back into the house, the water leak caused the casing around the hole to swell up tightly around the AC... this made the AC rest horizontally and become STUCK in the hole.
Needless to say I was stuck too and learned an important lesson. Don't let them frame it too tightly. Good weatherstripping will insure a nice seal around the unit. The AC unit has to be able to be removed easily (from the inside) and be able to drain.