Pick a sweater to build your project. I'm using an old sweater I've had since eighth grade. It's pretty heavy cotton and not very stretchy. The sleeves are falling apart a little but I'll cover that up.
Seek out garments that are sturdy and not too delicate-- these components amass to about the size and density of a checkbook-style wallet, so lightweight cashmere might not be a smart choice.
Cut a kangaroo pocket from fuzzy fleece fabric-- it's very forgiving. Pictured is Cuddle Fleece from Fabric.com.
Turn under the edge and pin the pocket to the front of the sweater.
You can stitch the pocket on by hand or use a sewing machine. Remember to only stitch the top and bottom edges of the pocket to the sweater, and then stitch a hem by hand around the pocket openings.
If you want to embed switches in the sleeves, you can cover them in fuzz too. I wanted to cover up the deteriorating sleeves on this old sweater anyway!