Cut out a long rectangle of fabric.
Fold in half so that the wrong side of the fabric is facing out and sew up either side leaving about 3 inches at the top.
Cut two long lengths of string or ribbon.
Lay the first length of string down so that it loops around the top of the sack.
Lay the other length of string facing the other way, so that you have two ends and a loop at either side.
Fold the excess fabric edge over the strings and pin tightly in place.
Sew below the string on either side of the sack.
Fold inside out.
Tie the two ends of the tails at either side in to a knot.
Pull together and you've made a drawstring.