Clean your juice pouch:

Slit open the bottom of the juice pouch using a pair of scissors and rinse in warm soapy water. You don't want your coin purse to be sticky. Let dry before sewing.

Sew on zipper:

The reason why you want a 8" (or more) zipper for a 4" pouch is because you have to open up the zipper and sew the zipper on each side of the pouch separately.
Once you've attached the zipper with a couple pins (optional, but make sure you're pinning it in the same line that you're going to sew because the holes will show!) sew a straight stitch 1/8" from the zipper.

Alternatively, you can skip this step and just stick on a piece of velcro.

Sew the sides:

Next you're going to sew up the sides. Simply fold the pouch in half, make sure the zipper is within the two sides, and sew up the sides with a straight stitch (or zigzag stitch) and a 1/8" seam allowance.

Trim Off Excess

Using a pair of scissors, cut off any excess zipper, loose threads or unaligned pieces of the juice pouch.

Finished!

You now have your own juice pouch coin purse!

If you want a clear ID slot, you can cut out a piece of clear plastic the same size as half the juice pouch, and sew the bottom before the zipper and the sides at the same time as the juice pouch.