I'm not sure if I know but I just did a ton of shirts and made every mistake possible.
If you have trouble peeling the back off of the shirt, before you cut out the design, make a tear in the back of the paper and rip it enough to rip just behind your design. KWIM? That way, after you cut around it, you can just pull the torn part off of the back and rip the rest off of it. That worked for me. For the designs that have square edges, I was able to split the back from the transfer on the corners pretty easily but I have long fingernails that are strong so it may not work for you.
I'm not sure about the wrinkling. I didn't have any problems with wrinkling. Except for the one that I had to iron an additional transfer over the previous one that I goofed up and put my child size design on my husband's shirt. It was a huge tee shirt with a design the size for a four year old.

Yeah...I made every single mistake possible.
I did iron the shirt on the front and back prior to putting the transfer on it so that it would be smooth. Maybe that helped?
I did find that the transfer directions said to iron the transfers for way longer than you needed to. When the tissue paper adhered to the transfer and stopped sliding around (slightly), I knew it was done.
We're leaving in two weeks and my shirts are DONE!
