There are machines in various places all over WDW. You put in 2 quarters and one penny, and the penny gets "pressed" with all different characters on them. Usually, each machine has one character that it makes, on some machines there are multiple choices.
They also sell books with small clear pockets in them to save your pennies and display them in. All three of my kids bought one of the books, they are pretty neat to save the pennies in.