If you plan on creating a T-Shirt quilt, it is highly recommended that you interface them prior to sewing.
Interfacing is a material that adds structure to flexible fabrics. Using interfacing is key when working with T-Shirts because of how flexible the cotton T-Shirt material is, it ensures that your seams will stay straight and your fabric won’t stretch.
Working with interfacing is very easy. I buy a bolt of Pellon Shape-Flex interfacing from Joann’s. (40% off coupons come in really handy for items like these!) One side of the interfacing is smooth, and the other bumpy. Those bumps you feel are the adhesive that will stick to the fabric once heated.
Although it does create a teensy bit of waste, you should cut your T-Shirt just slightly larger than your desired cut size (so desired block size + 1/2″) along with the interfacing. Iron the interfacing onto the back side of the T-Shirt and then trim the shirt to the proper size. Applying the adhesive in this way serves two purposes: 1) you won’t accidentally overlap the interfacing on your cut-to-size shirt and a) get glue all over your ironing board and b) won’t have to trim off any extra interfacing and risk damaging your shirt; and 2) it ensures that your interfacing will be at the edges of your shirt, and therefore in your seam allowance making sewing easy.
My father-in-law made me a set of T-Shirt Quilt Block Plexiglass Templates. These templates include the 1/2″ seam allowance and are marked with an X which tells me where the center is.
The example below is from my brother’s T-Shirt Quilt, Greek Eagle.
Here’s the How-To for Interfacing T-Shirts:
If you are using the method I do, the shirts need to be cut in increments of 4″. If you are not using pre-cut templates, do not forget your 1/2″ seam allowance!! Choose the size block that best fits the shirt you’re using.
Trim your shirt slightly larger than the desired block size. Cut interfacing to the same size (as best you can).
Turn the shirt so that the pattern/logo is face down. Place the interfacing bumpy side down on top of the shirt. Iron on the interfacing until it adheres to the shirt.
Now trim your shirt to the desired block size (with your 1/2″ seam allowance!).
Note: Interfacing is not a requirement, however I would greatly encourage it.
Sew You Think You Can Cook 
I’ve had certain types of interfacing melt to my iron when I do it this way… it may be better to iron fabric side up instead, or use a presscloth to make sure that doesn’t happen!
Are you ironing glue side up?!! If you put the fabric up any accidental over lap would definitely stick to your iron. The only time I ran into a problem was when I put glue side up–disaster.
No, I had it the right way. It was Pellon, but I don’t remember what kind – it’s super thin stuff, and just melted directly on the iron – what a mess!
Oh that does sound awful! I’ve only used this version because it’s what my mother-in-law has found to be best.