Learning how to chop a top is much easier on a pickup than most others. I ususally have some rough idea how much I want to chop, but then as the project progresses I let the size of the windshield opening and head room determine what the final cut is going to be. Toughest part about a chop is keeping the vehicle proportions correct. Mail slot sized windows look pretty dumb on a full width, full length pickup, etc.

Before you cut anything, make sure you brace up the cab just behind the doors to prevent it from shifting around. Some peices of 1" square tubing in an X between the door jambs will hold it just fine. Get good measurements on everything before you cut, then as you are going back together check these measurements to make sure nothing has moved.

Mark your cuts on the rear of the cab and the A Pillars using a template made from poster board, then line them with masking tape to make the cutting easier. Using a template will help you to make the cuts on both sides the same size, and the same angle.

Other than that, the books referenced here are a very good source of information, and having a donor cab around is usually much easier than having to fabricate the roof and door peices you will need to fill the stretch when the roof comes down.