You should end up with as smooth and round a hole as possible. Check it with a magnifying glass, if possible. Sand off any burs that remain, and dust. Tape the foil with the pinhole over the hole in the oatmeal box. The pinhole should be as centered as possible from top to bottom.
Electrical tape works well to secure the foil in place.
Ideally, the foil should be taped inside the camera, but this is not critical. A shutter will also need to be devised. A piece of opaque black paper taped over the hole is good enough. It will need to be easily removed or flapped open for the exposure. Don't tape it directly to the foil, as it will rip when you try to remove the shutter.