there is an actual pattern for t-shirts quilt- i'm at work so can't look at mine to see who made it- but you can do a search---you will find many=and i believe there may be a tutorial here on the board--
but in a *nut-shell*
you collect the t[shirts you want to use-
you purchase lightweight Tricot fusable interfacing-
you decide what (decals) you want to use on the t-shirts and cut them out- into squares or rectangles- what ever works---press the fusable tricot to the back of the t-shirt decals-
lay them out in a pattern that appeals to you- filling in here & there where needed with pieced cotton fabrics-you can use plain fabric- make blocks to fit areas, do it how ever you like. piece it all together- put it together- backing, batting, top--quilt and bind.