Start by measuring your waste and noting down the measurements. Cut a rectangle of fabric measuring 12" x the measurements of your waste + 1/4 of your waste measurement.
Flip the fabric over, so that the wrong side is facing up and fold the top edge over so that it meets the 6" mark.
Fold up the bottom of the fabric so that it also meets the 6" mark.
Now fold the bottom of the strip up, so that it meets the top and pin in place along the top edge.
The measurement of the belt should now be 3" high.
Sew along the open edge, removing the pins as you go., but stop sewing 3" before you reach the end of the belt.
Fold the middle of the bottom in, so that it creates a slant toward the middle of the fabric, repeat with the top so that the end of the belt forms in to a triangle. Fold under the edge and pin in place.
Carefully sew along the triangle edge, removing the pins as you go. Make sure to sew along the bottom edge too, so that the triangle keeps it's shape.
Turn the belt around, tuck the opposite edge under and stitch up the side.
Get yourself a buckle, you can pick one up in a craft store or use one from an old belt. You don't want to have a fastening clasp on your buckle, so if it has one, remove it using pliers.
Slip the square end of the belt through the buckle, so that 2" are sticking out the other side.
Flip the belt over, and fold the 2" of the belt back, pin in place.