These instructions are cribbed from the far superior ones at Cleo's plastic bag site. But then hers aren't done by a dog.
First of all, grab a bag (preferably smelling of something tasty)>
Then cut off the bottom of the bag.
Straighten bag out> And fold not quite in half - leave an inch at the side.
Fold in half again, then neaten the top by cutting off the handles...
Next - cut through the layers in inch or inch and a half strips but do not cut the inch that you left at the edge. Just like so..
On this picture, I've drawn in some black lines on the uncut part of the bag to show you where you need to cut next - this is the tricky bit. ('specially when you've got paws).
And here is the finished ball of plastic ready for Tutmut to knit with...
Now aren't I the clever one?