What you need:
1. Hot Glue Gun & Sticks
4. 41 PENNIES
First, cut your cork to fit a penny square of 5x5 pennys, 25 total.
Second, start to hot glue the pennies to the cork square.
Third, after you have finished gluing the first layer of pennies on, glue a penny square of 4x4 (16 pennies) in the middle of the 5x5 square.
Fourth, cut the visible cork from around the pennies so it looks nice and smooth.
Fifth, set coaster down, and put drink on coaster. TA-DAAA