This is a simple way of cutting up plastic bags, ready for knitting without having to tie lots of knots. 
1. Take a plastic bag and wash it.
2. Cut off handles and the bottom of the bag.
3. Fold it from one side to the other but not all the way.
4. Make cuts, leaving one side free.
5. Open it all out. Now cut the strips into one continuous strip by cutting diagionally from the end of one strip to the beginning of the next one along on the other side.
You can knit with it, or, spin and ply the plastic, into a fine yarn, or bring it to PYF and we can do it for you. 
Knit useful things in plastic as much as you can. Here is a film about plastic pollution. 
and here is a u tube demo incase you don't understand my instructions.