First take your can of choice and clean it out of the excess soda ( you can air dry it now or dry it with a paper towel later
Step 1: Cutting
Cut the top and bottom of and dry out the inside. Cut as high and low as possible to have as much material as possible. Then flatten it with the part you want on top. Cut the back part off.
Step 2: Measure
Measure out your coaster size mine is 3" by 3". Then flatten it as much as possible. Then round out the corners.
Step 3: Grip
I use hat glue but you can use what ever you want. Scratching it will let the glue grip better on the metal. It doesn't have to be pretty, no one will see it.