What you'll need:
1 lonely-only sock (you know, the one the dryer ate the mate to)
1 plastic grocery bag
1 tablespoon catnip (optional - some cats just aren't into it)
Cut a piece out of the plastic bag (about 1/4 to 1/2 the bag, depending on how big you want the toy to be) and stuff it into the toe of the sock. This gives it that nice "crinkly" sound that cats love. Add the catnip (if using) and shake it up a little to spread the catnip throughout.
Tie a tight knot in the sock, as close to the "ball" as possible.
At this point, you have several options - you can leave the top of the sock attached as a "handle" so you can dangle it for your kitty; you can cut the top off close to the knot so your cat can play with it alone; or you can cut the top off and tie (tightly) a piece of string, yarn, or elastic around the base of the knot at the top of the ball to hang it from a doorknob. Any way you do it, your cat will love it - and you!
(Note - the pics are not exactly actual size, the toy is about 1-1/2 inches across and 2-1/2 inches long, including the knot, using 1/4 of the bag.)