Replace a door panel outer skin
A genuine vehicle manufacturers doorskin will usually be for the full door from top to bottom, aftermarket parts will normally be the lower section up to window level, and much cheaper. Lower door skin repair sections (rust repair panels) are available for a lot of common models, but the extra work involved cutting, welding and filling, is not usually worth the few pounds saved, especially long term.
Remove all the trim, locks etc. and particularly glass to avoid spark damage. Use an angle grinder with a metal-cutting disk to grind about 2-3mm from the edge of the doorskin . This cuts through the folded outer skin without removing much metal from the door shell. If only the lower half (to window level) is being replaced mark carefully where the joint will be, and cut across the outer skin with a hacksaw.
Replacing the door skin
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
Fitting the door skin and preparation
When the skin is firmly crimped in place, a few spot welds in the original places will suffice to hold it securely, as well as a seam weld if the window surround section has been hacksawed for a lower section replacement.
Grind flat any welds, using filler where required, and also check for ripples and damage on the outer skin edges where the crimping has been carried out.
Sand or wire brush any loose factory primer from the door edges and apply a coat of etch primer on the inner folded edges *before* a bead of seam sealer is applied to the inner door seam. Check that the drain holes in the door bottom do not get blocked with sealer
Etch and primer surface the full door. Colour coat the inside of the door, then when dry fit it to the car before spraying the outer panel. ( If it's a solid colour and you are sure of a good match, paint the whole door before fitting. Apply heavy tape to the edges for protection before refitting)
Don't forget a coat of Waxoyl or something similar to the insides before replacing the inner trim - most rust on replaced panels starts on unprotected inside seams!.
After all that, one last thing to consider; check around locally for salvage breaker door prices - you might just find a complete door with glass for the price of a doorskin, never mind the paint etc.!