You will need approx. 1/2 yard of lightweight fusible interfacing per shirt. I am going to be cutting twelve 14.5" t-shirt blocks. So, you'll need about 6 yards of fusible interfacing for this quilt.
Cut out the whole front of the shirt- along the side edges, and along the top of the shoulder. You should have 1 layer of shirt now with your design on it.
Cut an 18" square of your lightweight fusible interfacing. Lay your t-shirt, design side down, on an ironing surface. Place the interfacing on your shirt, making sure the fusible side is touching your shirt. Iron the interfacing to the shirt. Make sure all areas of the interfacing get enough heat/time with your iron to securly adhere it.
Flip your shirt over right side up on your cutting mat. Cut out a 14.5" square, centering the design. If you are going to make alot of these, a template is helpful. I had the lumberyard cut a 14.5" square template out of plexiglass. Then I could just plop the template down & cut around all 4 sides. It's not necessary, though- you can also just use the lines on your cutting mat.
Prepare 12 quilt blocks in this manner.