1.) Clean the underside of the rim with a nylon brush and homemade cleaning solution of vinegar, baking soda, and water.
2.) Shut off the water supply and flush the toilet to empty the tank.
3.) Block off the siphon jets with duct tape.
4.) Pour 1 gallon of vinegar in the tank and flush the toilet. Let it sit over night.
5.) In the morning remove the tape and open the water supply. Flush water through once to clear out any remaining vinegar.