Unfold a bag and lay it out on your work surface. Cut the bag into 2-3 inch wide strips, the full length of the bag. You don't have to be perfect about making the cuts perfectly straight, just try not to make any jagged edges. If you have sharp sizers, you can start a cut, then slide them up the length of the bag.
You will probably need about 10-15 bag length strips, but it depends on how careful you are at stretching the material in the next step.
Cut each bag length strip into 8-12 inch sections.