Step 7: Backing, gaps, and (optional) pockets
Measure the back side, and cut your backing t-shirt to fit.
(Make sure you leave a bit of extra fabric for the hem - about 1/4 - 1/2". I serged my edges because i could, but if you don't have a serger a zigzag stitch will prevent the edges from raveling, or you can just be extra-careful about folding before you hem.)
Pin it onto the base fabric. Pin the center as well as the edges - you don't want a bunch of puckers in the center. Think of this as basting, as if you're quilting, and give the shirt a few runs across the center with your sewing machine to really get it situated properly.
Fill in the gap between your previous t-shirt strips and the backing t-shirt with more strips. I cut them from my backing t-shirt to help it blend in a bit better, but you can certainly continue with your original color(s).
Now is also the time to add internal pockets, as the order of operations (sewing on pockets or backing t-shirt first) will vary depending on the size and location of both your pockets and your t-shirt's design. I did the (probably too-big) big internal pocket first, then had to wrestle with sewing down the shirt without making the pocket unusable. To top it off, I added another pocket on the OUTSIDE - extra positioning fun. Plan better than I did.