I believe you are on about the rear quarter panel.
Very straight fwd to change, no bracing required, just make sure the car is nice an level.
The panel is spot welded so to start with remove the the quarter glass and rubber seal. Offer up the new panel over the old panel and mark two lines, one on the B post (the one directly behind the door) and one on the C post. DO NOT CUT TO THESE LINES.
Support the car evenly on axle stands on the side you are going to remove and then remove the rear wheel.
Now on the rear seam between that joins th rear panel to the quarter panel carefully remove the seam strip. This will allow you access to the spot welds here. Now using a spot weld removal drill bit carefully drill out all of the spot welds on the rear seam up to the lines marked. Do the same to the window aperture up the B post and then along the bottom of the quarter panel and up and over the arch down to the small lip at the rear. You will need to remove the trim strip there as well.
Now back to the two lines you previously marked. Using a 1mm slitting disc on your grinder cut below these lines by approx 15mm. You can trim up to the cut line later.
Now very carefully using a good quality thin scraper or chisel gently pry the old panel away. You may have to redo some of the spot welds you have drilled. Right on the bottom just behind the door cutout there is a small angle to the quarter panel, there will be at least one or two welds there as well. remove these.
Now you should have the old panel off. Offer up the new panel and check your cut lines on the B and c post. Now you can do the final trim to allow the panel to butt weld to the pillars.
With it all off you will end up with this

New panel clamped in place
