To prepare "plastic yarn":
Flatten plastic shopping bags to original shape and fold lengthwise until you have a strip approximately 1 � to 2 inches wide.  Using sharp scissors, cut off bottom of bag and top handle.  Cut remaining bag into 1" to 1 �" segments, depending on the type of shopping bag you are using.  Lighter weight bags require wider segments.  Open each segment into a loop, and slip-knot segments together to make a continuous 2-ply "plastic yarn."
Hint: to crochet with "plastic yarn," a loose tension is a must!

To crochet tote bag:
Beginning at the bottom of the tote bag, make a chain loosely to desired width of bag.  (Hint - 39 stitches = 15" wide bag) Single crochet (sc) into each stitch on one side of chain, then sc x2 into end stitch.  Continue to sc on the other side of  the chain, again sc x2 into end stitch.  Note: sc x2 at each end only on first row; this will allow work to fold into a bag.
From here on, single crochet into each stitch from the previous row, working round and round to desired depth (height) of bag.  Hint: to ensure a better shaped bag, fold work after the 3rd or 4th row and continue crocheting with bag in flattened position.

To make handle:
Single crochet across approximately 1/3 of the width of the bag, then chain to approximately 1/3 the width of the bag, then continue to sc remaining 1/3 width of bag.  Repeat for other side.  Now continue to sc into each stitch, including chained stitches.  Depending on the size of the bag, I suggest four rows to make the handle.
If you wish to make a shoulder bag, crochet a strap and attach on the sides of the bag.

To make "squared-off" base:
Turn bag inside out and sew across the corners of the bag, using "plastic yarn" and a large darning needle.  Do not cut off corners.  Turn bag right side out.  This technique gives a more "finished" look to your bag.