The heelflip is one of the more basic flip tricks, but just like the kickflip can be extremely hard to learn. Some people find heelflips easier than kickflips, but in the same respect others find kickflips easier than heelflips it just depends on the skater. Make sure you can ollie properly before you try heelflips.
Description: The skater pops the tail and kicks their front foot towards the toe side rail, causing the board to flip 360 degrees in a sort of barrel roll.
For a heelflip you need to have your front foot about two-thirds up the deck with your toes hanging slightly over the toe edge. Some people prefer twisting their front foot a few degrees in either direction - for instance I prefer to have my foot facing slightly towards my nose instead of flat across. Your back foot should be in the middle of your tail with only the ball of your big toe being used to get the pop (I find it helps to have your back foot slightly to the heel edge of your deck because it helps to centre your weight above the board). It's pretty much just an average ollie position as far as you back foot placement goes.
Roll along at a speed you are comfortable with (probably quite slow at first), crouch down, and pop your tail down hard like an ollie. Straight after you pop, almost in the same movement, slide you front foot up the deck and flick it almost straight off the nose, except slightly off the toe-side corner. Jump straight up and keep the board directly under you. Depending how hard you flip you tricks it may rotate fast or slow, everyone does them differently. Ideally you should be flipping your board so it is ready to catch at the peak of the flip. Wait for it to rotate completely, and catch it with either your back foot or both at the same time. Stomp it down hard over the bolts (hopefully), and ride away.
If you're having problems keeping over your board as it flips, here's a couple wee things to try. As you crouch down to pop, keep your shoulders locked in and your back straight - sometimes this corrects a tendancy to jump forwards and miss the catch. Also, try keeping your whole body parallel to your deck throughout the entire trick. Otherwise, just keep practicing, and if you need any more help email me.