I am sorry for your loss. I've made several t-shirt quilts here are my tips:
1. interfacing Pellon 911 (use a coupon at Joann's n buy the bolt)
2. get a teflon (or silicon) applique sheet. This lays on top of your ironing board, place the t-shirt face down on it when you iron the interfacing on the back. The sheet does 2 things, it stops the decal/printing/picture on the t-shirt from melting into the ironing board; and, two it keeps the gunk from the interfacing from getting on your ironing board.
3. to clean the iron after you are done, place a sheet of bounce dryer sheet on a cheap piece of fabric (muslin) and while the iron is hot run it across the sheet/fabric layer, do this a couple of times using each sheet only once, it will clean almost 100% of the stuff off your iron.
good luck and God bless