Flatten bottle caps by using a hammer and weight. To flatten the cap, lightly tap the edges of the cap with a metal surface at an angle. This will make it flatter without getting rid of its shape. Once you've done this, place some heavy item (e.g., books) over it to avoid any change in position while it "sinks" to your desired thickness--usually about three millimeters for bottle caps used in craft projects or even a little more if you want them to be thicker for things like coasters or jewelry lids.