The process of cutting is easy. Just get a hot knife and slice right through the middle of every pillar holding the top on. You then need to determine how far down to drop the top. You'll want to keep as much of the pillars (those still on the top, and on the actual body) as you can because these joints will be sanded and CA'd. The hard part is when the top is dropped so far that the pillars won't line up anymore. Since that is just about when the "chop" starts to look cool anyway you will generally have to section, ie stretch the actual roof by slicing across the top two or three times and then glueing in braces along the underside of the roof. The gaps between roof sections are filled with putty but the gaps have to be narrow so that the putty actually does its job. Also fill in putty around each pillar joint once they are glued. A lot of sanding then follows. Door panel lines are hard to scribe into the completed pillars because of the small glue joint. But if one is careful it can be done. Here is one I did recently.
