Grab a 4' level, a sharp razor knife, and a pencil. Using the level and a pencil mark out the hole you want to cut, then place the level along one of the lines and using it as a guide make a couple light passes with the knife (don't press too hard). The key here is to be very careful, and take your time. Once you have shallow scores in the wall the knife will follow the same path through the wall on each pass, so you want to make sure these first cuts are straight and in the proper location.
Continue in this fashion until you have all the lines scribed with the knife, then start making deeper cuts until you have cut all the way through the wall (you should not need the level at this point, just be careful to follow the scribe lines when cutting.
If it's drywall, you could use a keyhole saw to make the cuts after scribing the lines I find the keyhole saw makes a more jagged cut, which requires extra finishing with a rasp or sand paper. I prefer to take a bit more time and use a razor. If you do use a keyhole saw, you'll want to make sure you know what is in the wall (electrical, plumbing, etc). The keyhole saw could easily damage an electrical wire, and give you a nasty shock. If this is a plaster and lath wall, the keyhole saw could make a much bigger mess since cutting through the lath can be problematic.
If the wall is covered in drywall, you're done. Hole complete!
If the wall is plaster and lath, you'll have to cut the lath out without damaging the remaining plaster. I find a Cut Out Tool like this
Works very nicely for cutting lath. Unlike a traditional saw that cuts with a back and forth motion, these cut using a spinning bit so it's less likely you'll vibrate the remaining plaster off the wall. You'll want to be careful with this, as you could easily wander and damage the surrounding plaster ruining your nice straight line. You could use the cut out tool right from the start, but you'll need a special bit when cutting plaster.
