I love pockets, for kleenex, keys whatever but most pants now don't have them. So I bought a pair of pants with faux pockets that have working zippers and plan to transform them.
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.
The picture to the right is what the underside of the stitching looks like. Now sew the other end of the pocket to the upper welt and around the edges to form the pocket. I used a very stretchy knit and it's hard to adjust the pressure foot pressure on my sewing machine (new to me Bernina 820) which everyone says is great but I prefer my late Kenmore 70s machine. So the fabric is wavy, but no one sees it so I won't change it.
So now from the right side you can see the pocket pulled out.
You can make one or two pockets, since they don't change the looks on the outside it's your choice.
And this is a foolproof method, no matter what happens you can always keep the pocket zipped close and no one is the wiser.