https://frenchsleepdeprivationstudy.blogspot.com/2009/06/snack-bag-tutorial.html
It's a free pattern, just click the pdf link
I haven't tried it yet but it's one of those things that I keep meaning to do when I ever find the time. Especially since dh goes through his weight in ziplocks and conveniently is a chip junkie.
Re: Link for turning Mylar bags into reusable snack bags
What a great idea!