This project was relatively easy and cheap. I found plain zip-up hooded sweatshirts and some yellow felt.
Rip the seam of the hood out down to the seam that attaches the hood to the rest of the sweatshirt. Skip over this seam and cut the sweatshirt down the center to the lower seam.
Place the edge of the triangle even with the cut edge of the fabric on the sweatshirt and sew about 1/4 inch in from the edge.
When you are almost to the end of the first triangle stop sewing. Make sure that the needle is down and lift up the foot. Pull back the top layer of fabric and place the corner of the next triangle against the corner of the last. Lay the fabric back over the top, put the foot back down and continue sewing. Do this with each triangle until you get to the seam of the hood. Sew up to the seam, and back stitch about 1/2 inch.
Move to the other side of the seam and continue adding triangles down the back until you get to the bottom seam. Finish off the same as you did with the hood.