I am no stranger to carrier bag knitting, but past attempted have all involved tying whole bags together, which is adequate for making hammocks or rugs, but not so good for more conventional knitting. So other techniques are required.
Here's my step by step guide to knitting with plastic carrier bags.
I use ones from Sainsburys, they say on that to reuse them, I am. They also say there are made from 50% recycled material. Whatever I knit will be from 100% recycled material.
To turn them into 'yarn' first pull them through your hands to make them crumpled into long thin things, handles at one end, cul-de-sac at the other.
Now use scissors to cut them widthways into 1 incg chunks to make lots of crumpled rings. You can chuck away the cul-de-sac end and the handles end. I can't think of any use for these bits.
Now unfold all the rings that you've just cut. Some of them might have split, chuck away the broken ones. There's probably a really neat use for reject bits of cut up carrier bags, I dunno what it is.
Now join all the loops together, where you loop one ring into another them pull it back through itself. You're smart, you can figure it out. Don't pull it too tight mind, its not going to be going anywhere so it doesn't need to be really small knot.
That's you done with the 'yarn', I guess you can roll it into a ball or something. And then its just a case of knitting as usual.
I think I'm going to make a shopping bag of some description. I'm using 7mm needles bought for 20p from a charity shop in Burnham-on-Crouch, in the past I've bought very cheap knitting needles from Oxfam shops and very expensive needles from John Lewis, whatever meet your price point I guess.
For this shopping bag I casted on 40 stitches, its probably a wee bit too wide. I did a trial run of 16 stitches and that looked less than half of the size I wanted. Suck it and see, and try again. I'm doing that knitting thing where its just row after row of purls rather then knit one purl one.
I'll let you know how I get on.