Heelflips are a lot like kickflips. The only difference is that you spin the board the opposite way. Put your back foot like you would for an ollie and your front foot with your toes hanging off the front of the board. Try putting your foot in the middle, and then try moving it back and forward to see which is easiest for you. Do just like you would for a kickflip, except kick off the heel side of the board. Ollie, and when your foot gets to the end of the board, kick out hard. Wait till it gets done flipping and land griptape up.