1. cut out a rectangle of fabric. yup, it's that simple! you don't even need it to be any particular size. whatever you want. but for those of you who like a little more direction, i usually aim for about 8" by 10"... obviously the size you want is going to depend on what size your bag is b/c you don't want the pocket bigger than your lining. also, you don't want the pockets so deep that you have to dig around in them for your phone or keys or buzz lightyear action figure.
2. fold the fabric in half (right sides together) so that it measures 8" by 5". sew around the three raw edges, leaving a space to turn.
3. turn pocket right-side-out and iron. then you'll want to stitch along the top edge. (i recommend using the long edge that you stitched, as opposed to the folded edge, b/c then you'll have less bulk to sew through when you attach the pocket to the lining.) also, if you have a tag you'd like to put in the bag, this is a great place to attach it.
4. center pocket on lining fabric, about 2" below top edge. (again, this is mainly a personal preference, but i find that if you attach the pocket much lower, it becomes inconvenient to reach into your bag for things.) pin pocket in the center along the bottom edge to mark place.
5. to right of the center pin, fold over fabric and pin pleat. repeat on left side of the center. pin around the rest of the bottom and side edges so that pocket lays flat.
6. starting at the top (and close to the edge of the pocket fabric), stitch pocket to lining around the sides and bottom. back-tack a little extra at the beginning and end for extra hold. then stitch a line down the center of the pocket, also taking care to back-tack at the beginning and end of stitch.
hey, you've got a pocket :) now you just need to find buzz...