I ended up making two blankets. After the first one I made some modifications to make it even easier. I'm going to share the directions I used for blanket #2, which I like better. Each blanket used approximately 30 t-shirts.
1. ~30 shirts. You only need 30 designs for this size, so you can use backs and fronts of the same shirt. Just make sure to cut the back and front apart before starting step 1.
2. Large piece of fleece or other sturdy material. Same size as your t-shirts. Mine was 5'x6'
3. Sewing Machine
6. 12x12 template