I don't really have a shed, but I am an uncle, so here's how I go about shoving copper foil inside a guitar
The victim is the Heartfield EX2 recently refretted (in another thread). The cavity is painted with shielding paint from the factory but there's absolutely no continuity anywhere (ie. it doesn't work). This guitar has an active circuit with a fair amount of unshielded wiring so needs a well shielded cavity to keep out noise.
My shielding foil isn't wide enough to do the entire cavity floor so I'll be doing that in 2 strips. First I make a rough impression of the cavity shape in the foil by pressing it against the cavity edge.
Then I trim the foil slightly oversize to ensure it runs up the cavity wall slightly. This gives something for the side wall foil to overlap. If there's a really complex bit, like on the top right here, I'll trim that exactly to size to keep things tidy.
I start to apply the adhesive foil. It's often helpful to start with only part of the backing removed, peeling off the rest as you go.
Arriving at this kind of result
The pots and switches in this one sit in slight recesses. I cut an X in the foil over each recess to help it press in without too much tearing. In retrospect I should have gone Chris on the cavity and sanded off the painted-in tearout around the holes. You can see that in the top right mini switch area the foil has torn on the uneven recess edge. I might end up removing the foil in the circular recesses completely. The body of each switch and pot is connected to ground anyway, so gives us an effective shield in those areas already.
Then I do the second strip, overlapping the first, and prepare the side wall strip. I take rough measurements of all the flats and curves, add them to get a total length, then cut strip of foil about an inch longer than that. I then measure the wall height and cut that strip so that it will sit slightly higher. Cutting it now makes it easier to apply.
I then roll the strip into a cylinder shape and start applying it, removing the backing as I go.
I give it a little overlap at the ends and trim off the excess.
Relief cuts at the corners of the top edge allow it to be folded over. I like to trim off the majority leaving just enough folded over where at least one cover screw will force it into contact with the cover shielding.
Trim the foil anywhere a cable needs to pass through.
Most shielding foil has conductive adhesive which will join the pieces electrically where they overlap. If your foil doesn't have conductive adhesive, or if you're stupidly thorough you can use a little solder to join the copper. This cavity has 3 pieces, 2 on the bottom and 1 around the side wall, so I find a spot where all 3 pieces meet and solder them together. You don't need any more than a small spot.
Later I'll put the wiring back in with fresh switches.
Do you run a fly lead to the back of a pot for ground or just trust the pot chassis' to conduct well enough to ground?