Once the decision is made on exactly how much of the panel is to be replaced, measure exactly where you want to make a joint across the pillars, and mark it accordingly on the vehicle and also the new panel. The same process of finding, noting and removing spotwelds applies as for front wings. Most will have been hidden by window and door seals, probably with some more obvious welds around rear lamp clusters and boot panel joints. Lower edges of wheelarches are usually spotwelded along the lipped flange, but some models may simply be glued.
Before attempting to cut the pillars or sills, it is worth cutting an "inspection" hole a little further down, to check for reinforcement sections which may be directly behind. If so, more care must be taken, cutting only the outer panel exactly on your marked line. A sharp hacksaw is probably best for a neat cut. Once the panel is removed, clean up all edges to remove weld stubs, metal tags etc.
