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.
There have been times when my stabilizer has not remained fused to the fabric I’ve adhered it to. In that case, I simply spray a little bit of basting spray, and that does the trick.
Do not iron directly on the T-shirt design. You never know if the design will stick to the iron and ruin both the T-shirt and your iron.
I always cut a piece of interfacing slightly larger than the T-shirt. This ensures I will be able to use as much of the T-shirt as is necessary.