Start by getting the water out of your toilet bowl. Turn off the water supply to the toilet (usually a small valve on a thin pipe leading to the bottom of the toilet tank), then flush the toilet to drain the water in the bowl. Turn carefully; the valve may be stiff since they don't get turned off and on very often,
Put some warm water into your pail, pull on your rubber gloves and scrub the rust stains using your sponge and the scrubbing cleanser. This will remove some of the rust stains that won't come off with a toilet brush.
Now it's time to do the heavy duty work and use the pumice stone. Wet the stone before using it to scrub the stains. A pumice stone is very abrasive and harder than the rusty water stains, but fortunately, it's not harder than the porcelain toilet bowl, so the pumice stone won't scratch the bowl.
Continue dampening the pumice stone and scrubbing--hard. You will slowly be able to get rid of the rust stains, but it will require some time and hard work.
Once the stains are gone, don't forget to turn the water supply to the toilet back on.