you need a fusable stablizer (not paper backed fusable used for appliques---a leave in one sided fusable stablizer---found at joannes & stores like that)
you fuse the stablizer to the back (wrong side) of the motif's/designs you want to use - cut them out- into the sizes you want them to be- lay out your t-shirt pieces in a (pleasing) layout- fill in around them with blocks- strips-what ever you choose to make them all go together into a quilt top-
there are many tutorials and commercial patterns for t-shirt quilts- just do a search