First thing to do is to zigzag stitch the fabric under the zipper to make sure they don't run when you slit the fabric. I did one pocket before slitting the fabric and the other pocket after. Before works better because the fabric is stretched out, afterwards, it rolls and hard to sew. I used Fray Check
at the ends because a sewing machine can't reach the corners. Here is it after slitting and stabilizing the edges.
I cut 2 pieces of fabric the width of the pocket welt 4.75" in my case and about 10" long. First stitch the pocket fabric to the lower welt, see below.
And this is a foolproof method, no matter what happens you can always keep the pocket zipped close and no one is the wiser.