Settle the Score
To kick things off, you'll want to remove any embellishments from the wall in question. For the sake of minimizing a ridiculous mess, lay down a sheet of plastic first. Put on your safety goggles, then meet you new best friend, Mr. Crowbar. Pry bars are great at pulling off trim, paneling, and any other accoutrements your wall could feature.
The next thing you want to do is get a utility knife and score the outline of where you want to cut. Using a level to make sure your line is straight is a smart move. Otherwise your project could end up as crooked and bent as an M. C. Escher drawing. Take several passes with the utility knife to make the line nicely defined.