You cut them into inch or inch-and-a-half strips (thicker plastic = narrower strip) right across the width of the bag, discarding the bottom seam and the reinforced bits at the handles, to make circles of plastic yarn (plarn) You loop the circles through each other to make a continuous doubled "yarn" Crochet hook size 10mm at least.
Tesco have gone back to putting online orders into plastic bags since lockdown. They use cheap plain white ones - which are better for crocheting than thicker ones. Their meat packs and/or dairy items are in red bags. I found some coloured ones stored away at the back of a cupboard - green or turquoise from M&S, orange from Sainsbury's, a large golden yellow one with red logo from Argos. Bin liners are a plain black. Some brands of bin liners are too good though, they are too thick to work with.