Felting pad or brush (or piece of foam)
(You can find all the felting tools at most craft stores)
Heart cookie cutter
and push it down through the roving, the sweater and into the pad below. The teeth on the needle will grab a few fibers of roving and push it though to the other side, entangling it with the sweater fibers. Keep punching away, paying extra attention to the edges.
After about 5 minutes it’ll look like this. Now remove the cookie cutter and touch up any of the edges that aren’t clean by punching the needle around the exterior. If there are places where the felting is thin, you can pull off a little more roving and punch it into the thin areas.
Pull the sweater off the pad, the back will look all fuzzy. Take an iron on the wool seating with steam and iron the front and back of the heart to set the fibers.
Wear your heart on your sweater with pride.