Based on the pillow measurements, cut the back pillow pattern pieces first. There will be two pieces overlapping (envelope-style) in the middle of the pillow. The preexisting hem on the bottom of the T-shirt will serve as the ends. Cut the front and back of the shirt’s hem the same width as your pillow, and three-quarters the length of the pillow.
Cut the remaining top of the shirt (the graphic), the same exact measurement as your pillow (length x width). This will be the front of your pillow. You should now have three pattern pieces: the front of the pillow (the graphic) and the two back pieces (the hems).
Take your other back pillow piece and place it on top of that, right side facing down. Place this on the opposite raw edges of the pillow, so that the hems overlap in the middle. Pin around all four edges and then sew. Use a zigzag stitch on your machine to avoid the knit from stretching.