Close inspection of the ground-through edge will reveal the outer skin and doorframe layers as you cut. When all the edge is removed peel back the outer skin. On later models there may be a side impact bar or a stiffening strip fitted which may also be glued to the outer skin. There will probably be a few spot-welds to finish off with the grinder, and some adhesive in places. Once the outer skin is gone, peel off the remaining inner strip and clean up any rough patches with the grinder.
Use a hammer and panel block (or solid rubber block) to even out any damaged edges on the door shell before dropping the new skin over it to check for fit. Once the new skin is aligned properly, the edges must be folded over tightly using a hammer and block. This takes a bit of practice to get absolutely right, so it might be worthwhile trying some spare metal offcuts first. Support the outer skin and doorshell with the block held at an angle so that only the edge is supported, with minimal contact on the outer surface of the skin, to avoid damage to the outer skin. Using a flat faced hammer (preferably a panel hammer) bend the inner lip tightly against the doorshell, working slowly around the door.
Door skinning tools are available, ranging from simple "crimping pliar" types which can leave marks on the outer surface, to more expensive ratchet/roller models, but for a one-off job a little bit of patience with a small amount of surface damage is probably preferable
