A properly set up foundation is extremely important for the absolute best results. No matter how tempting, don’t just dive right into doing body work. Starting in the right place is key, and will make everything else much easier (and better).
The place to start is really figuring out where all the high, lows and panel edges are; and making all the proper adjustments. We will assume the body panels are adjusted as close as possible.
You will need flat stock and c-channel sticks of aluminum. These will be used to span over the panels, simulating where the final body work will be.
The aluminum stick you’re using should take just a slight amount of tension to follow the shape of the vehicle. If it’s too rigid, it will want to bow out too far, and too loose, it will follow the low areas too much. This will take your judgement to decide what feels right.
Use as long of a piece as possible, and move back and forth and up and down a lot with it; and really study where the high areas are. If something needs to be shrunk, or if some larger low areas need to be worked up (if possible).
Door edges and fender edges get adjusted here as well. Every panel edge must be touching the aluminum; the goal is to have thin metal edges everywhere once body work is complete. This is the step to achieve that. Hammer edges out, or cut, or do whatever is necessary to make sure your edges will be thin. It is ok if an area like a quarter panel is a little low, as long as the filler won’t be too thick, and it’s impossible to tell how thick the filler will be.
Attached is a video showing how to check the side of a truck. All of this will take some judgement and decisions on your part, every project is different. But this step is very crucial!!