First you'll need a plastic bag, the size dosen't matter at all! :-)
Then you're gonna fold it like this.
And fold the other one too into the middle.
Then fold the bottom once, just like a triangle.
Fold it normally, like this.
This is what it now would look like.
If the bag is too long, then fold the top normally like a sqaure.
It should now look like this.
Then fold the square into a triangle, just like you did the first time.
Place the "new" triangle into the old one, if you "open" the old triangle you will see that there is a opening to place the "new" triangle.
And here's the final result, simple and it's now more easy to get your plastic bags organized.