A heel cutting is a way of getting a cutting with a bit more plant matter at the bottom where the roots will form. This gives it a better start as there's more plant matter to get it off to a good start.

Heel cuttings are usually taken from side shoots on stems that are more than two years old. What you need to do is to pull the side shoot downwards from the main stem. You should end up with a "heel" of the older plant from the stem attached to the basal end of the side shoot. Imagine ripping a branch off a tree and you end up with part of the trunk attached to the branch. This is exactly the same but on a smaller scale.

The heel cutting can also can be cut from the main stem with a knife. Put the knife blade in at the top where the main stem meets the shoot. Cut downwards and make sure you take a sliver of the main stem with it.