Here I am about to start sampling some goodies off the smoker.
First a safety warning. I used old propane tanks for my cook chamber and firebox. Propane is VERY dangerous. You should seek a professional to render the tank safe for construction. Do not attempt this yourself.
Step 1: Door cut outs
The door/doors design is a personal choice. Mark and cut whatever you feel will work best for you. I elected to make two larger doors with the little door in the middle (shown cut out in this pic) so that my cook racks could be one piece, slide out, and I would have room to place a whole hog in it without fighting to wiggle it in and around a center piece.
A word of warning here. Don't be surprised if when you cut the tank things warp a bit. This is normal and not your fault. There are ways of correcting this that I won't go into now.
Once you have your doors marked you can cut them out with and acetylene torch, plasma cutter, hand grinder with cut-off wheels, or a sawzall with metal cutting blades. I have also known people to cut them out with a circular saw with a metal cutting blade. I prefer to cut with a some sort of saw as the gap is less in the cut and I don't cut a very straight line with a torch and can't afford a plasma cutter.
Another tip is to not completely cut out your door. Leave some materiel in the corners and middle of the top and bottom of the doors to keep the tank in alignment for as long as possible. You will go back and make the final cut outs later.