The seam allowance is the cloth between were your needle sews and the edge of the cloth.
Stick the 2 pieces of Velcro together and Fold the back seam allowance flat Velcro and all. And sew only the bottom edge of both pieces of Velcro to the inside of the BACK
Now, you should have one side of the BACK with the Velcro face up and one on the side side that you folded over with the seam allowance with the Velcro facing down.
Open the slit in the back , and sew down the inside edge of the seam allowance .
Now just hem the bottom along hem 11