Firstly you need two coke cans with their inside of their tops cut off. Use the tin snips to cut them, this will leave a lethal jagged edge which you must clean up either using a metal file or a small flap sander (could also use a Dremel)
Then cut the bottom off the cans using a Stanley knife. Try not to crease the metal as this will reduce the chances of it being airtight. Some people say that you can use a can opener to remove the tops of the cans however I found that it destroys the sides of the can, you might have more luck!