Step 1: Lay Out a Plastic Bag.
Lay out a plastic bag, tucking in the side seams and smoothing the bag flat. It doesn't have to be perfect.
Step 2: Cut Off Very Bottom of Bag With Scissors.
Step 3: Fold Bag.
Step 4: Cut Folded Bag Into Strips.
Keep it simple.
Step 5: Unfold One of the Cut Strips and Begin Joining.
Grab a second cut strip and unfold it. Loop it around your thumb and index finger and hold the rest of it in your fist.
Step 6: Continued Bag Joining.
While maintaining the loop in your right hand, grasp the strip from your left hand and pull it partially through the loop in your right hand. (The 2nd & 3rd pics in this step.)
Step 7: Finish the Bag Joining & Snug the Knot.
Grasp the other end of the strip that was passed through the loop in your right hand with your left thumb and index finger. (2nd & 3rd pics in this step)
Draw the part you've pinched in your left hand through the loop on your left wrist. Continue pulling the knot tight. (4th, 5th, & 6th pics in this step)
Here is a short video of how it works:
Step 8: Finish Joining Bag Loops & Begin Crocheting.
The resulting yarn is best crocheted with a size K crochet hook (or larger). See the pics in this step to see how I started crocheting a door mat and then continued adding strips to the end of the 'yarn' to continue crocheting.
Step 9: Finished Crocheted Shopping Bag.
The pattern for this bag can be found at: <http://orangeflower.typepad.com/orange_flower/2008/02/crochet-basket.html>
I have been informed that the afore mentioned link is no longer valid. Please try this one: http://chickpeastudio.typepad.com/chickpea_sewing_studio/2008/02/the-little-croc.html
I crocheted my round until it had 64 stitches around. I then kept crocheting those 64 stitches in the round until I wanted to do my handles.
When you get to the point where you want the handles: Place marker and single crochet 7 sts, chain 18, count 18sts from where you did the 7th single crochet and slip stitch to here. Single crochet 14 stitches. Chain 18. Count 18sts from where you stopped the 14th single crochet and slip stitch to here. Single crochet 7sts. Join with a slip stitch to Beginning Single crochet (by marker). Chain 1 and turn. Do at three more rounds like this for strong bag handles.
Remember you're carrying groceries in this.