Items you will need:
1 - No. 10 Can (Standard restaurant size can. Mine was a pineapple chunks can from a local pizza parlor. They gave it to me free.)
4 - Regular soup cans (Standard 8oz Campbell's Soup cans)
Insulation material. I used Perlite. It's cheap and available at any garden center. I had it on hand for my garden. You can use sand, dirt, ashes, foam, any insulator.
Hammer and nail (to punch starter holes)
Pliers (makes bending the metal a little easier, but not totally necessary)
Step 1: Mark and Cut hole in No. 10 can
Take one of the four soup cans (soup can #1) and trace the shape onto No.10 can. I cut my hole slightly above the bottom of the can to make it easier to avoid dealing with the bottom of the can when cutting and shaping the hole. Seems to have worked out fine.
I then punched a number of holes into the circle so I could get the tin snips in there to cut the circle out. Punching that many holes turned out to be a waste of time as a single hole would have been fine. The tin snips cut through the can quite easily.
I used a pair of needle nose pliers to bend the edges of the hole back to get the final shape, but in retrospect I would have probably just relied on the sharpie outline and cut the hole wider to begin with. I was somewhat conservative when cutting and widening the hole took some effort. Later can fittings were made easier by just cutting the hole to the right size to begin with. I was afraid of cutting too big to begin with and therefor spent a considerable amount of time fidgeting with it.