The pattern I use for the pocket was from this pattern, Simplicity 5541:
You can take the pattern for the opening and the pocket flap and put it in any handbag that has a lining. Here’s what the pocket lining looks like; I have cut a template for it from posterboard and cut out the opening for the zipper (on the template only, you don’t cut it out on the fabric) so I can easily transfer the markings:
Notice that one side of the pocket lining is longer than the other, it is shorter below the opening. When cutting out the piece from fabric, you would just cut out the big rectangle. The hole for the zipper on the template is only used for marking the stitching line. You mark the small, long, narrow rectangular opening on the bag lining and the pocket lining:
The pocket lining is placed on top of the bag lining, right sides together, matching up the zipper opening lines, with the smaller half of the pocket lining on the lower portion of the bag, and the longer half sticking up over the top portion of the bag lining:
The pocket lining flaps will drive you crazy unless you pin them out of the way. I learned to use safety pins after I endured many injuries from straight pins. You can see the safety pins in the photo below. Then you stitch around the zipper opening with a small length stitch:
Pin the flaps out of you way again and insert the zipper. I used to use pins and basting and who knows whatall, but I found that if I just tape the zipper in with tape on the back around the edges of the zipper, then topstitch around the front of the zipper opening, it takes about 1/3 the time. I do a double row of topstitching to hold the zipper tape down neatly on the underside of the opening.
Now you are done. If, however, you goof like I have done in the past and put on the pocket lining with the shorter end up, then insert the zipper and, by gumbo, refuse to rip out the zipper and everything to get it right, you can just seam the pocket lining bottom, press it so that the pocket is the right size, but the seam is on the pocket back, and continue with the side seaming. Because it’s inside a pocket, for Pete’s sake, who is going to know?!?