I used appx. 30 plastic bags for this bag, crochet needle 6mm. The size of needle that works will depend on how wide you cut the strips. I have figured out a way of cutting the plastic bags into one long uninterrupted strip.
collect enough plastic bags to make a bag, I used around 30, just regular shopping bags. If you want a nice fluent design you need to pay attention to the colors. Lay them out on a table or on the floor. Look at how you want your gradient / stripes to work, one bag makes between 2 and 3 stripes. Most of the bags are white from the inside, so that will wash the colors out. Put your bags in chronological order, which bag first etcetc. I.e. I decided to use my black bags first, so that the bottom of my bag wouldn't show dirt immediately.
check that all the bags are clean. If they are not wipe them off with a cloth, or maybe even wash them (by hand) and dry them. also check if all bags are in good condition. Some bags may be a bit decomposing, that won't work so nicely anymore, as the strip will then break all the time.
making the plarn. Now cut the strip, this is the most difficult part! First lay the plastic bag down flat on the table. Check the bottom of the bag, there is a sort of fold in the bag, cut that ply open up around the side is probably necessary, so that you really have the bag flat on the table. Then fold it double, and again from the middle fold over one more time, it should not go all the way to the side leave around 3 cm free. do this twice if necessary.
Now cut the "roll". I used a scissor, the knife in the picture is just there to add weight. cut slits through the roll, but leave the side-space untouched (the 3 cm) I cut my slits with appx 2 cm in-between. If the plastic is thinner or softer you can make it wider, if you are working a thicker very solid bag, make it less.
Now roll-out the bag and cut loose and trash the handle-bit as well as the bottom-bit. Now we need to make the slits into one long strip. On one side of the bag, continue the slit all the way through, then the bag will look like this:
And now here follows the trick. Take hold of the side of the bag where the slits have not been cut through. Study it carefully, place an imaginary diagonal line between the end of one slit and the beginning of the next. You now need to cut from slit to slit, every time along this imaginary diagonal line. Be careful to not cut through some of the other strip by accident. Finish the strip by cutting a little diagonal cut out the border. Now you have turned one plastic bag into one long uninterrupted strip!
crocheting the bag. Crochet the bag like you would do with any other crochet work.
Mine is simply a chain of 25 stitches,
2nd round; 3 hf cr stch into first loop, then half cr.stch into each loop, 3 hf cr.stch into last loop, back again by half cr.stch into each loop.
3rd round; 2 hf cr stch into first loop, 1 hf cr stch, 2 hf cr stch, countinue to the other end and make the corner in the same way you did the first, and crochet all along the side.
4rd round; 2 hf cr stch into first loop, 3 hf. cr stch, 2 hf cr stch, continue to the other end and make the courner in the same way and finish the other side.
from here just continue in rounds.
Leave a space open of 9 stitches, crochet back and forwards 3 rows, cut the strip and start on the second half. upon finishing the second bit, re-establish the lost bit of the handle by looping 9 stitches in both holes. Continue in rounds again.
make a knob by crocheting a little roset, use a piece of elastic band to fasten to the knob.
Featured on Instructables on 24th of January 2012