Its really easy :)

Step 1: What You Need

•an old knitted jumper (or anything thats knitted)
•scissors
•needle and thread
•a slice of sponge
•felting needle
•felting wool

Step 2: Sew It

Cut a rectangle out of the old jumper. Then fold up the bottom like in the picture and sew it in place, the turn it the other way out so the stitches are inside. Also hem the other edges.

Step 3: Felt It

Put the slice of sponge directly under the side you are going to felt the design on, this is important otherwise you might accidentally felt it shut.

Use wisps of felt to build up your design and stab then in with the felting needle each time until they are like part of the fabric.