THIS IS NOT MY CONTENT
this Korean site for those who have seen the "100+ Free Zipper Pouch Patterns" post on The Sewing Loft (this is pattern #90). It is similar in technique to a much larger, waterproof shower bag pattern on the German-language blog LeileLou - check that one out if you're working with laminated fabrics or prefer a cleaner top seam finish (which would be difficult if not impossible at the scale of the coin purse).
For general hand-holding WRT zippers, try this free Craftsy class.
For the triangle change pocket you need:
1. Lining 20x12cm
2. interfacing 20x12cm
4. Zipper 10cm
5. 35cm twill tape
7. Wood Button
1. Affix interfacing to fabric, sew the zipper to the short sides
2. Close zipper, turn inside out, stitch bottom. Box corners @ 4cm
3. Turned to the front, bottom sewn:
4. Apply top edging, sew on loop of tape and wood button as a handle.
Triangle change pocket completed!