If you are not sure how to needle felt or where to get supplies from check out my blog post here. The principles are the same as when needle felting an object but instead you matt the fibers into the cardigan or jumper.
Make sure you have your foam block underneath the area you are felting and design away. Make sure you turn the cardigan inside out and needle felt from the back as well to secure the design.
I also went over mine with a bit of embroidery thread to add some detail.