The idea is simple and the picture explains it well enough without tutorial. The zippers can be brought in bulk from amazon or ebay or if you are really frugal, you can remove the zipper from old clothing. The zipper can be either glued on or if you would like to take some more time, you could sew it.
The cases can be made in all different sizes and colours making this a cheap way to store small things.
Here is some of my other favourite plastic bottle upcycles.
Tutorial on FabArtDIY
Tutorial on The Frugal Crafter
Tutorial on Rust and Sunshine
Tutorial on Creme de la craft
Found on Just Imagine
Found on Recycled Crafts
Found on Jo-ann Fabric and craft stores
Tutorial on Filth Wizardry
Tutorial on Living chic on the cheap