Here's how do it: Place your feet as if you were about to ollie, but with your front foot a little more towards the toe side of your board (let you toes hang off the board, with only your heel on). After you set up your feet, snap down on your tail, letting your front foot slide towards the nose, then off the toe side of your board, causing the board to flip. Hover over the board, wait until it has flipped once, and then land on the trucks. That's it!
There are many people who perform heelflips improperly. Many people don't concentrate on the pop, only on the flip, causing the board to flip on the tail. If you do this, the nose will end up in your ass, so don't do it this way. Make sure you concentrate on the pop as well as the flip in order to good heelflips.