I use a Pellon type facing for my shirt quilts.
~Cutting the t shirts out roughly and my pellon out nearly to size. (I put a template I cut out of plexi glass over the shirt pattern so I have it centered correctly)
~fuse the pellon to the shirt (lightly)
~cut the block to size, I use a 1/4 " seam
~then I dampen the muslin, shirt side up, and set it in.
I believe the fusible is for the sewing part, it may losen up in the process, can be re heated. When quilted the fusible helps with the stretching found in t shirts.