I have made 6 Harley T-shirt quilts. Here is how I do it:
I use my 12 1/2" square ruler to cut out the imprint on front and back.
I then fuse a 12 1/2" lightweight fusible interface on them so they don't stretch out of shape.
For a queen size quilt I do 20 squares in the middle arranged 4 across 5 down with 2 1/2" finished size sashing (3" cut strips) between each square. I do cornerstones as well. So I end up cutting 3 x 12.5 for sashing. I sew on each side of square (or top and bottom) and then sew a 3" square cornerstone on each end of the 3 x 12.5 peices of sashing I have not sewn to the t-shirt square. I then make my rows. Row 1 3 and 5 have sashing around all 4 sides of squares. Row 2 and 4 only have the sashing between, nothing on top and bottom. I then sew these rows together. Then I take a complimenting strip cut 1.5 and do a narrow inner border (this is optional). I then take more T-shirt squares and sew them together with no sashing to make my outer border. If you don't do an inner border the quilt will take 7 12.5 squares sewn together for top and bottom and 8 12.5 squares for sides. If you do the inner border you will need to cut one wider square for each side to make up for the extra 2" you added with the inner border. I just completed a top recently that I have to sandwich and tie. I will try and get a picture of it on for you to look at. Harley T's make great T-shirt quilts because they have such great colorful imprints on them.