Visit your local fabric retailer and purchase half a yard of a thick cotton blend fabric in your preferred color.
Lay the fabric on a flat surface in front of you. Measure and draw a horizontal line 4 inches long on the fabric with a measuring tape and fabric chalk.
Measure and mark the halfway point along the horizontal line. Measure 5 inches straight down from the half way point you just marked and place a second mark with fabric chalk.
Draw two diagonal lines between each end of the horizontal line to the second point you marked on the fabric. You should now have a triangle shape.
Repeat Steps 2 through 4 to create a second triangle shape with exactly the same dimensions on the fabric.
Cut the triangle shapes out of the fabric with sewing scissors and place the remaining fabric to one side.
Put one piece of fabric on top of the other. Pin the fabric together along each edge of the triangle with sewing pins.
Place a sewing machine on a zig-zag stitch. Sew around the triangular piece of fabric with the sewing machine.
Measure hips of the person who will be wearing the strap with a measuring tape. Subtract 1 inch from this measurement.
Cut a piece of flat elastic in the length you just calculated. Lie the piece of fabric in front of you so the triangle is upside down. Pin one end of the flat elastic to the top right edge of the fabric and the other end of the elastic to the top left edge of the fabric with sewing pins. Make sure the elastic does not twist when you perform this task. This is the waistband of the athletic supporter.
Measure and mark the two points on the elastic waistband that are 2 inches to the right of the right side of the triangle and 2 inches to the left of the left hand side of the triangle. These points determine where the elastic for the leg straps will sit.
Measure the girth of the wearer's upper thighs with a measuring tape. Subtract 1 inch from this measurement. Cut two pieces of flat elastic in the length you just calculated. Pin one end of one piece of elastic to the bottom right hand side of the triangle and the other end to the point you just marked on the right hand side of the elastic waistband. Pin one end of the other piece of elastic to the bottom left hand side of the triangle and the other end to the point you just marked on the left hand side of the elastic waistband. Again, be careful not to twist the elastic when you are performing this task.
Sew the waistband and the leg straps to the fabric with the sewing machine. Sew slowly and remove the pins as you go.