Though I've seen it done on the vehicle before, I suggest removing the door(s) from the vehicle and placing them on a sturdy work surface. Though it isn't as important during disassembly, you'll want to make sure you protect the new panel from damage during the installation phase, so position the door on the work surface with that in mind. It's also a good idea to at least remove the door glass-welding and grinding will produce hot metal that melts into glass and plastic, so removing the window glass and any trim or speaker components is also a good idea.
The original skins are attached to the inner door assembly by crimped and spot-welded edges. The skins, both original and replacement, are slightly oversized so that the outside edges can be folded over and attached in that manner.
The easiest and fastest way to remove the old skin is to grind the edges. The skin is folded over, crimped, and spot-welded to the inner door assembly. Grinding the edge wears away the outer skin at the fold.
This image shows a portion of the bottom edge that's been ground. You can see where the skin is separated from the door. Use caution though-you don't want to grind away too much, just keep your eyes (protected with safety glasses) peeled and move on as soon as you begin to see the gap.
Orlando used a sharp gasket scraper and hammer to begin separating the skin from the door. Sometimes there is corrosion that holds the skin to the door in some spots. This is a good way to pop the skin loose.
Here you can see the portion of the doorskin that remains after grinding. The strips of metal will be removed after the skin has been taken off.
At the top of the door there are supports spot-welded to the inner doorframe. These support the top of the skin at the window opening. They are detached by drilling out the welds.
At the rear top portion of the door (above the latch assembly) there are usually multiple spot-welds. Look closely and make sure you've got 'em all so you don't end up wrestling with a hidden weld.
It's worth the time to go around the perimeter of the door one more time looking for spot-welds you may have missed. The trick is removing the skin without bending or distorting the edge of the inner door.
Once you're confident everything's loose, carefully lift the skin off the door assembly. Remember, the edges are going to be sharp so use caution.
Here's the inner door assembly less the outer skin. We're halfway home.
Using a grinder, and a light touch, clean up the edge of the inner door to remove whatever corrosion or rust there may be so that the new skin will sit flat on the edge.
After cleaning up the outer edge, Orlando flipped the inner door over and began removing the strips of leftover skin on the backside of the inner door edges.
Some of the spot-welds popped loose with the gasket scraper wedge (see previous image), other more stubborn holdouts had to be drilled out.
There were some portions of the leftover strips that Orlando pealed off using a pair of locking pliers-saving him at least a few fingertip cuts, I'm sure.
Here you can see the new doorskin lying next to the old. Re-assembly is about to begin.
The next order of business is to position the new skin on the inner door assembly.
Orlando uses welding clamps around the perimeter to hold the skin in position during the beginning of the re-attachment process.
A hammer and dolly are then used to slowly fold the skin edges over the outside edge of the door assembly. Work slowly and carefully; you don't want to damage the face of the doorskin. Once the entire perimeter of the skin has been well-crimped, you can then grab the MIG and add some spot-welds of your own (don't forget the upper supports in and near the window opening).
And here's the finished product, just like new! A professional like Orlando can complete a chore like this in an hour or so. Though don't expect times like that on your first or second door-remember, slow and easy, and it'll be a piece of cake.