A comprehensive tutorial with advise from a real carpenter. To start... Check both sides of the casing for plumb with a 4-6 ft. level. Measure an inch wider and taller than the existing casing ( if the opening is way out, you may need to go wider) or 3 inches wider than an opening without casing. That way if the casing isn't perfectly plumb, it'll hide behind the door. You don't want to be able to see behind the door through the gap either. A customer complained about a door that I didn't build and asked me to fix it right.