Last time I did this, I did it in four parts:
1 - cut out the outline of the bottom of the hole in the copper, then stick it down
2 - measure the depth of the hole, then cut a copper strip that wide. Stick this around the edges (trimming as appropriate). make sure that it overlaps the bottom piece.
3 - solder the edges to the bottom in a continuous bead. A little practice would not hurt to make sure you don't turn your guitar into a wood burning kit.
4 - cut out another piece for underneath the pickguard/control cavity cover. Stick that on, drilling holes as required.
Solder a ground wire to the appropriate pieces and you are done!
Note that I don't bother with this any more. It's just not needed IMHO.