Towel Halter
I got this one from teenmag.com along with the picture.

Materials:
Old beach towel
Eyelet kit containing at least 9 eyelets (get this at the fabric store) An eyelet is a little metal circle that reinforces a hole in fabric so that you can lace something through it. The kit contains a bunch of rings (front and back) with a little puncher that you slam with a hammer to join them.
3 yards thin rope - any color.
Hammer
Sewing machine or needle and thread
Pins
Scissors
Measuring tape or ruler
Pencil or pen
Mirror
Wood scrap or other flat surface that you can pound on with a hammer
Procedure:
1) Measure the space from the base of your Adam's apple to your belly button (or wherever you want the bottom of your halter to rest). Add 3 inches to be safe and cut a piece that size off one end of your towel.
2) Stand in front of a mirror and hold the piece up to your body (good side to skin). Center the top edge with the base of your Adam's apple.
3) Fold the corners down from the center so that the towel lies comfortably under your armpits and still covers all "the goods," then pin. Your towelette should look like a pentagon now.
4) Stitch along the folds with your sewing machine or needle and thread and trim off the excess fabric.
5) Hold the top back up to your body and pin up the bottom to the length you like. Hide your bellybutton or show it off. It's up to you.
6) Return to the sewing machine or needle and thread and stitch it up.
7) Trim off the extra.
8) Read the directions on the back of the eyelet kit and practice a couple of times before moving to the fabric.
9) Try to strike the pin square on the head to fold the teeth evenly. And always make sure to use the hammer on a scrap piece of wood or another parent approved surface or you may never get to show off your new halter because you'll be grounded all summer!
10) Place one eyelet ring at the tip of the pentagon, and four on both parallel sides, 9 in all.
11) Thread your rope through the tip. Bring the ends around your neck and criss-cross in the back all the way down like a shoe!