"Waist stay" sounds like a terribly technical thing, but really it's not. It's just a ribbon about 1/2" smaller in total than the waist measurement on the skirt itself. You sew it in, really just on the seams, put in a hook and eye (or a very flat button) and voila - waist stay.
A picture is worth 1000 words.........
This is the waist stay I put in my new denim skirt when closed. Do you see much difference in the way it closes things? Of course not. (You do see the interfacing. Always interface your zippers, even on heavy fabrics).
Here it is buttoned with the waist stay open. You can see how it's just a smidge larger than the button closure, making for a nice snug fit.
I weighed the denim skirt in question the other day. Five pounds. Five pounds of fabric, hanging off my waist. Yes, it *does* slide if it's not given a bit of help. This little ribbon is my "little help". :) You wouldn't think a hair ribbon from the dollar store would do that...
I find as I read further in "couture" sewing that mostly it's just about the differences in craftsmanship and materials. You really can do quite a few little couture touches at home, and they're not particularly difficult to do (at least one at a time!). Fine craftsmanship is always worth the work.
No, it's not an overnight thing, learning any craft - but I find that it's worth it.
Such a small detail to make such a difference.