1. Are you sure you want to? Don't you have a friend with a dog who needs the 300 million plastic bags under your sink?
2. Really? You don't want to try fusing it so you can sew with it? Look, here's a tutorial from Etsy! Oh, who am I kidding, fusing plastic bags sucks too.
3. Okay, that's not fair. Fusing plastic bags & knitting with plastic bags are both fun. Infuriating, but fun.
4. Here's where I'm at so far in my [possibly insurmountable?] knit shopping bag project:*
Let me tell you, those first few rows suuuuuucked. But then it got less sucky; my shiny silver thrifted knitting needles helped, for sure. (Though the silver is starting to flake at the ends, which makes me sadface.)
5. That's actually pretty much the only advice I have--the first few rows suck ass because plastic bags have zero fucking give, so every damn stitch is a struggle. Ha, you can tell I'm annoyed, look at all those curse words! But really, the first few rows suck, but if you keep going (and try to keep the tension as loose as you can**) it gets much less annoying. So stick with it! (Or quit, whatever makes you happy.)
6. There was one other recommendation I had, but fuck if I can remember it... I've been cutting pretty wide strips of plastic, which I think will end up making the bag more sturdy. There's a bunch of good tutorials for how to cut bags and prepare them for use as plarn (pardon me while I vomit, but that word just makes me crazy). Really though, just cut off the handles and the bottom seam and then cut them into strips that are 1" wide. Or wider, or maybe less wide if that's what you're into. Tie those strips together and start knitting. I do a slip-knot*** kind of thing that makes a pretty seamless knot. Oh, and use grocery bags! You could probably use other bags, but grocery bags are so easy and you or someone you know has 500,000 of them cluttering up their house.
7. Also, you should be smarter than me and find an actual pattern to use instead of just winging it. I really need to learn how to increase stitches, I bet that would make it way easier to knit a shopping bag without a pattern.
8. I think that's it?
*I am not even going to show you the terrible things I made when I tried fusing plastic, but trust, they were awful. I have been considering trying it again though, I have a seriously ridiculous amount of plastic bags under my sink.
**Unless you usually knit with a low tension (loose? knit loose? is that a thing?), then probably just proceed as normal. But seriously, plastic has zero give, so do yourself a favor and keep the tension loose.
***Did you hear one of the guys in Slipknot died? I saw that on some gossip site and spent at least seven minutes reminiscing about my poor music taste as a youth. Oh Slipknot and [other band names redacted so you will still be my friend], why did I waste so much time listening to you?