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.
The creator of this guide has not included tools