The main thing in using T-shirts for the fabric in quilt tops is to be sure to stabilize the T-shirt material. The easiest way is a fusible stabilizer. I tend to cut widely around the T-shirt design I want to feature. I then iron on the fusible stabilizer before I cut the square or block shape desired. I then played with the layout of the blocks. As my blocks were not all the same size, I created a sketch and calculated how strips would need to be added so each row was the same length and each block within the same row was the same height.
Don't you love the block above with the three girls which states "cause you're there for me too!"? And below the one with the Eskimo and his dog?
My niece's high schools colors were blue and white. Her college colors were purple and gray. I chose to use the purple and gray. Purple fabrics are being used to even up the block size. Gray fabric is being used for the sashing between the blocks.
My niece has a very nice sense of humor.