See if this makes any sense to you...
This is my process. Since I sometimes use small t-shirts I don't worry about the size of the block, you can add a border around the small ones to match the size of your other Ts.
You need to begin by washing your shirts. Cut away the parts you want to use. Be sure to leave some seam allowance around the design.
I dont use any fusible web and the quilts have held up to use and washing fine.
Piece the rows together using a sashing between t-shirts. You can use shirts of varying length as long as the rows end up the same length. Make sure you have sashing on the ends of the rows as well.
Cut a strip of backing fabric the length of the finished piece + 4 inches all around.
Lay out the backing fabric wrong side up. Next lay on a piece of batting the same size as the backing.
Lay the t-short row on top of the batting and pin in place. Use plenty of pins to prevent shifting.
Now take it to your sewing maching and, using a walking foot, stitch the layers together "in the ditch" at the top and bottom of each t'shirt block. You are quilting as you go. Now add a lengthwise sashing on each side of the first row using the flip and sew technique, sewing through all layers.
On each subsequent row, only apply the lenghtwise sashing on the right hand side of the row.
Now sew the two rows together using by sewing the sashing from the first row to the left hand side of the second row, sewing through all the layers.
Turn the piece over, trim out one layer of batting from the sashing and the other layer to fit the sashing.
Trim the backing so the two pieces overlap by about 1/2 inch. Pin in place and hand stitch closed.
Continue with all the rows, trim and add your binding. You are done. No other quilting is required, but I have added some around the motifs for security and effect.