Just made a rear quarter patch panel for my '67 Dart.
First I made a template for the fore/aft shape of the opening, then another for the body side shape at the wheel opening and at the other end of the section I planned on cutting out (about 1" from the rear bumper)
Then I cut out the rusted section, the upper was easy, but the lower part is continuous spot welded to the trunk wing at the bottom where it turns under. I finally ended up just gringing it loose full length.
I used 18 Gage Cor-Ten steel sheet from Lowes, but any flat sheet metal from a hood, door or whatever will work.
I cut out the patch panel using the removed section as a template. The next part is hoakey, but it works. Not having a brake, I rolled the patch panel to shape with a piece of 2" pipe in my lawn, just stand on the pipe and roll it across the patch, it take a little time, but you can get the shape perfect.
Then I templated the wheel well lip and cut out a strip to fit. I welded this the patch panel inside and out.
Eastwood sells a panel lipping tool, but I made mine from a pair of vice grips and a two 1/2" x 1" steel blocks that I hacksawed and filed to shape (take a look at Eastwood's tool) and tacked these to the vice grips.
Whether bought or made, lip the patch and fit it to the car (unless you are REAL good and can make an unbacked butt weld). Tack it in place and finish weld it out with numerous small tacks (where I am now) to prevent heat build-up and warping.

Hope this helps