Ive just done one recently and this is how I tackled mine. I cut mine to suit rather than replaced the full length but the principals are the same. Unfortuntately I didnt take any pictures.
Firstly, most pattern door steps are awful. I got a magnum panel that actually fit and wasnt a bad press but ive heard others have a hell of a time with magnum door steps. That being said I still had to tighten up the folds and beat out a curve in the panel that stretched from one end to the other. I wouldnt recommend cutting before you have the panel in hand.
If youre only cutting the door step out and your inner and outer sill are solid you shouldnt need to brace the door frame but DO NOT jack the car up while it is missing the panel.
I started off by removing the door, rubbers and seals. I took out the carpet off the inner sill. I took the seat out and rolled over the carpet and felt too so I didnt damage it from welding spatter or grinding sparks.
Cutting out is fairly easy with a cutting disk. I cut the panel to suit along the edge of the seam. Youve then got a choice of either drilling the spot welds out of the lip or grinding them off. I ground mine off as I found it a bit easier. You also need to prepare the edges of the door step that is left. You can joggle as mentioned above but you can also butt weld, in which case you need to remove the paint for a clean surface to weld.
Now is a good chance to check the health of your inner and outer sill. Surface rust can be dressed and dealt with fairly easily. I had to repair a small section of the inner sill from a previous repair.
Now to prepare the new panel. If you havent got a spot welder you need to drill it out for puddle welding. I did my holes about 1 inch apart with a spot weld drill bit and then dressed. I made sure not to force the drill bit through so i didnt bend the panel. I also ground down the edges for clean metal for the seam weld.
Offer up the panel and make sure it fits. Having plenty of clamps/molegrips helps. Ive got 5 which did me ok. Once it sits nicely and it sits level then its time to weld. Tack it in and make sure its still square. Then you can start welding in. I have a spot weld clamp which clamps either side of the spot im about to weld. Means each spot is nice and tight. Id just say take it slow when welding. You dont want to overheat the panel and warp it and its a pain to cut back out a new panel if you mess it up.
I would say its one of the easiest panels to replace if you get a decent panel to start with.