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.
Use a pressing cloth (another piece of fabric) over the T-shirt design to iron. This barrier will allow for effective ironing without a potential mess.
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.
I always sew with the stabilizer side up, and the other fabric underneath. I’ve found it moves through the machine more evenly this way.