I wrote this for my own benefit (hence the rather patronising tone - I don't have a very high opinion of my own intelligence) but I figured I may as well put it up on here, seeing as it might be of interest to someone somewhere, if only as corroboration of the effectiveness of this method as written up by others in more detail than me.
1 Tube of Contact cement
A large pack of disposable gloves (they get sticky and you have to change them periodically)
Shielding Foil or Kitchen foil (the thick stuff for preference)
1 Roll painters tape (masking tape will do)
1 Sharp knife
1 Pointy Biro
If you are shielding pickup cavities which aren’t covered by a pickguard then use the masking tape to cover the surface of the instrument around the edges of the cavities. If you are shielding a control cavity covered by a metal plate on the front of an instrument (like a stingray or a telecaster) then mask the body around the control plate before you take it off. This is important as it prevents you getting glue and foil stuck anywhere visible.
Unscrew and desolder everything (and I mean everything, even the bridge - leave the neck on if you want though, and the strap buttons - they don’t really have anything to do with this) - you’ll need all the cavities clear of wires and objects that will get in the way – if you aren’t replacing the wiring as well then try, as far as is possible, to keep the pots, jack, and capacitors wired together when you take them out of the body (it’ll save time later) only desolder the bridge ground and pickup wires.
Once everything is out of the cavities lightly sand and then clean them as well as you can, try and make the surfaces as smooth and clean as possible.
Cut strips of foil to whatever width seems appropriate for the cavities you are dealing with, make sure that you have a good number of them ready before you start messing around with the glue otherwise you’ll have to cut them whilst all sticky.
You’ll want to do this next part either outside or somewhere fairly well ventilated - it won’t kill you to do it inside, but it will make you feel kinda woozy by the end of the job.
Put on the gloves (you really can’t do this job without them, I tried - it went horribly wrong and the skin on my fingers went all manky) and wrap your glue spreading finger inside something like a plastic bag or some paper and start spreading the glue evenly over the inside of the cavities, try and avoid getting any in screw holes.
Use the same technique to cover one side of a few of the foil strips. Pause for a minute or two to allow the glue to get its sticky on (the longer you wait, the stickier it gets – better for neatly plonking it on, but if you do it earlier you can slide the foil around to get it into the right place) then, without the bits of bag (or old gloves, or paper, or whatever you chose to use) on your fingers, start sticking in the foil, trying to get as little glue on your fingertips as possible. Ignore screw holes, wiring channels, holes for potentiometers etc - you can poke those holes through with a pen or something later on (if you can find them, try and remember roughly where they were - you can feel for them though if you forget).
This part is very boring and time consuming – just get comfy and sit around sticking the strips into the cavities until the whole cavity is covered with as few gaps as you can possibly manage. Change your gloves whenever the gloves become stickier than the surfaces you are trying to stick the foil to. Listen to some music by all means, but don’t watch TV, you need to be looking intently at what you are doing the whole time. You’ll want to take breaks from this every now and then, as your eyes will get tired focusing intently on the work and your brain will get confused by the fumes.
If you are putting foil into a control cavity with a plastic backing plate then try and get the foil to overlap the edges of the cavity into the bevelled (or countersunk, or chamfered, I have no idea what to call it – I’m not a carpenter) part under where the cover would be, complete the shielding of this cavity by covering the inside of the cover plate with foil so that it touches the parts on the bevelled section.
Once you have finished lining the cavities give yourself a manly handshake in appreciation of a job well done and go off for a while to get some fresh air and make the glue induced hallucinations go away. If you have been shielding cavities not covered by a pickguard or by a metal control plate then come back after about half an hour (after the glue has set fairly well) and cut around the inside edge of the masked part with a very sharp knife (very sharp!) and peel off the tape, taking the excess foil (and, as long as you've cut the edges properly, only the excess foil)
Now part one is done - I would write how to do the next part (wiring all the different sections of shielding foil to ground) but I’ve not actually done that bit yet.