I made up the heart as I went along, felting in more wool here and there until I was happy with the shape, but for something more structured you could draw a template first and use this as a guide. As a simple rule, the more wool you felt and the harder you stab it the more sturdy your decoration. Watch out for your fingers though, those needles are sharp!
Once you’re happy take some ribbon, tartan is obviously the best at Christmas time, and push a pin through the middle of the ribbon and down into your decoration. Tie the ribbon together at the top and hang on your tree! The perfect thing to make with a Christmas film on in the background.