I sewed mine too- I used fat quarters of fabric from JoAnns, Iron on fusible interfacing (2 sided) and ribbon. I needed fusible interfacing as the fabrics weren't very thick or strong. If you pick twills or something, you may not need it.
I know there are patterns/ tutorials around but I just made my own...
you sketch out what size you want. mine was for big pockets for circs, so it didn't have to be very tall. and only needed a couple pockets. if you want a long one, you will need a longer piece of fabric.
you pick out complementing fabrics- outside, lining, thread, ribbon.
since you said you sew, this isn't too detailed.... sorry if it makes no sense...
Iron on the fusible interfacing to the wrong side of one fabric
sew right sides together, 3 sides, and most of the fourth.
turn right side out, (poke out all the corners) press (pressing hem into open part.)
Fold up the bottom (narrow end of rectangle) to the depth you want for the pocket. (outside to outside. this is a good time to use the part with the opening) sew edges, turn right side out. you now have half an envelope. The top will be folded down for the flap.
sew into envelope part of bag for individual pockets.
attach ribbon to side edges
fold over top flap.