If you iron the interfacing on BEFORE you cut the t shirts, it is much easier, no stretch or curling edges.

..and cut the interfacing a little bigger than the square you are going to cut.

I usually cut an 18" square of interfacing out and iron it on my t-shirt back...iron it on....and then use my 15" template to cut my square...

I would have to agree with this. I didn't do that on my first one and had to re measure, trim and fudge after they were all fused. It was not terrible but there were places that did not measure correctly.