Smooth out a plastic bag and fold
Smooth out the bag and fold over and over. Cut of the sealed bottom and the handles.
Shake out your bag, fold again leaving about 3" at the end. Cut and odd bits of each end if you need to. Then cut about 1" strips (thumbs width) upto the top of each folded section. Shake out bag.
Put your hand through the bag and cut the first strip off. You will then cut from side to side until you come to the end.
When you have finished cutting, roll up your plastic into a ball ready for knitting, crocheting and so on.......