These are so easy to make. I place a 12 1/2" or 15" template over the graphic on the shirt and trim about 2" wider. Some shirts you may need to piece backs to front to fill up the "block". (these are plexiglass templates from a quilting store). Next: Stabilize the backs of the shirts, this is very important to keep them from stretching. I use Pellon 906F. After you stabilize trim to exact size using the templates. Now you have your blocks. Do a layout on the floor of how you want your shirts to look.
Then cut fabric 2 1/2" wide to sew to each side of the shirt and join them together forming a row. Then sew a longer strip to the top and bottom of the rows and join them together. After your rows are sewn together, add a strip to the left and right sides. Now you have a "top". I like to add a 1" border around that to separate from the 4" outer border.
If you have any questions let me know. Don't hesitate to do this!