I sewed hipbelt pockets by making a box, inside out, then inverting. So I had three pieces. I think it's more work than AndE's method, which I should probably learn. But corners are fun, in a challenging sort of way. The more you do, the better you get at them. Add your seam allowance to all of these:
-two identical front and back pieces, 4" by 6". I rounded the corners by tracing a glass jar. You can pick whatever length you want, I did one pocket 4x6 and one 4x8
-one piece for the top, bottom and sides, 2 inches by 28 (always add a couple of inches, because it's hard to estimate how much fabric you'll actually use around a corner).
Now the steps:
1) sew straps onto the back - I did velcro designed to fit in my MYOG pack
2) sew the zipper onto the front, along the edges
2a) cut a slit above the zipper
2b) fold the fabric in and hem next to the zipper
2c) attach a zipper pull
2d) fold fabric in and sew over the zipper ends
3) sew the pieces together, inside out. Swear a little when you do the corners. Cutting notches into the seam allowance on the long skinny piece (or either pieces) will help a lot.