The process is to turn your t-shirt design side down and put the stabilizer bumpy side down on top, then press with your iron until the stabilizer has fused to the back of the t-shirt.
How Much Interfacing do I need for a T-Shirt Quilt?
How Much Interfacing do I need for a T-Shirt Quilt?
This yardage depends on many factors, like the size of the T-shirts used. You may be making a quilt out of child-size T-shirts rather than a large adult so that would make a big difference.