The bottom and ends are folded, just like original, to make an original looking "invisible" repair.
At the top edge, its best to cut the panel a little lower and put a "set" or "joddle" into it so the new outer piece fits flush, this helps avoid distortion and reduces filling to a minimum.
Below, a front door that I did recently, the theory is exactly the same though, but in this case, I cut the door lower and put the set (which you can JUST see on the middle pic) in the repair section, so it stiffens that panel (the swage in the door stiffens the original) Then just drill or punch a row of holes in the bottom edge of the door skin where it meets the repair, plug weld through, dress and a skim of filler finishes it off. One more thing, I found it useful to cut a piece of wood to length to space the joint between the skin halves from the door shell, again, this helps avoid distortion in the panel.
There are 2 relatively inexpensive hand tools that are worth buying for doing this job, the first is a combination joddler/hole punch and the second is a door skinner, which is little more than a long handled wide ended pair of pliers with a flat block of some sort of plastic on one face to turn over and crimp the flanges into place, again without distorting the panel. Rather than welding or tacking the skinned panel back to the doorframe, I now use a good bead of Tiger seal to glue it on the flanged bit. I'll take some pix of my tools next time I'm down the shop and post them.
HTH, Steve