First, I picked out which shirts I wanted to use. I began arranging them to see what would be the best arrangement and size before I cut them.
After this picture was taken, I decided that I'd like to make the blanket a little bigger and used the backs of some t-shirts to add another column of 5 shirts to the blanket. Next, I used a cardboard piece, cut 14x14" as my stencil. I placed the cardboard on all the shirts where the main design was, and used a sharpie to trace the 14x14" square onto the t-shirt. I ended up with a stack of 20 t-shirts that were about 14x14" each. There were two shirts that I wanted smaller pieces, so I made them 14x7" instead. (2 of these made up one square that matched the others.)
Since these were all jersey cotton t-shirts, I used a fusible, non-woven iron-on interfacing around the border of each of the t-shirts. This prevents stretching and makes the sewing easier. You can use the interfacing on the entire back of the fabric, but to save money (and to avoid melting some of the designs on the front of the t-shirts) I only did the border of each square.
Then I laid out all the squares on the floor and rearranged them so that clashing colors weren't next to each other and there was a little balance.
Once I was happy with the arrangement, I used sticky notes and numbered the squares so I could remember where they were when I started sewing them together. Then I started sewing! After I got all the t-shirts sewed together, it looked like this:
I used a flannel sheet as the backing, sewed it all together, turned it inside out, stitched the opening together, and hand-sewed each of the corners of the individual squares together to keep the flannel sheet and the t-shirt together!