Take your time, I'm not making anything this week, but thanks for offering to help.

You start with a square of fabric, say 9", cut it in half so you get 2 half square triangles. One for the outer side, one for the lining (or make 2 from 2 different fabrics). Use some iron stabiliser if needed. Sew the zipper tape along the diagonal. Now fold the zipper tape in half and attach the pull, partly close it and stitch across the ends of the tape to secure. Then fold the whole thing flat, wrong side out, so the raw end of the zipper touches the top of the triangle forming a square and stitch the 2 open sides.
Then make two boxed corners in the usual way, about an inch deep.. Add lining made the same way.
Mine turned out to be approx. 6"x3"

Try it out with a paper napkin (without a zipper) - I did, hence the 9".

I do believe I understand so gonna give it a try asap. Thanks mops for being so helpful!

I bought zipper on the yard with a number of pulls and I manage well with those to get a 'normal' zipper, it is the one coil folded double that gives me trouble, especially when it is a short length.