Drill a hole in the base of the tin (so that the photo-paper can be easily put into and retrieved from the pinhole camera).
You should now have a hole- the size and shape doesn't really matter as the card with the pinhole will cover it and it means you can change the size of it easily if you wish.
The last step it to place the card with the pinhole onto the tin over the hole and tape it into place.
And its ready to use! simply bluetac photo-paper onto the tin lid and experiment with exposure time; then develop the images.