The wax ring under your toilet serves as a gasket between the toilet outlet in the base and the flange on top of your drain pipe. When the wax ring fails, water may leak from the bottom of the toilet when you flush, and you'll smell a foul odor. If you suspect you have a leaking wax ring, gently push to and fro on the toilet. If it rocks, the wax ring has failed. Allow the wax ring to come to room temperature before installation.
Turn the water supply off at the cutoff valve -- usually under the toilet tank where the supply line comes through the wall or up from the floor -- or at the water main for the building if there is no cutoff valve.
Remove the tank lid and set it aside. Flush the toilet and hold the handle down until the tank empties. Using a sponge, remove the remaining water from the bottom of the tank. Position a plunger over the toilet bowl drain. Push down sharply on the plunger to evacuate the residual water in the toilet trap.
Loosen and remove the water supply hose from the underside of the toilet tank, using pliers. Keep the sponge handy to sop up dribbles from the disconnected line. Inspect the rubber gasket in the end of the hose for damage or deterioration, and replace it if it looks damaged.
Remove the caps from the two toilet mounting bolts. Loosen and remove the nuts from the bolts, using pliers. Apply penetrating oil to the threads if they are stubborn. Use a hacksaw to cut the bolts off at the toilet base if the nuts are frozen and you can't break them loose.
Lift the toilet straight up and off the flange and mounting bolts. If the toilet is a two-piece model, be very careful not to strain the bolts holding the tank to the bowl, which can break the tank or bowl. Alternatively, you can remove the tank before pulling the bowl from the floor. Don rubber gloves, then have a helper rock the toilet back and support it while you scrape the old wax off the toilet outlet, using a scraper or putty knife.
Scrape the old wax off the pipe flange in the floor. Remove the mounting bolts from the flange, and remove any remaining wax. Inspect the flange and pipe for cracks or damage, which will need to be repaired before you reinstall the toilet.
Place the new wax ring over the flange with the wax side up and, if applicable, the plastic spout pointing down. Press the ring down lightly into place, then remove the plastic form from the ring. Install the mounting bolts in the flange slots.
Lift the toilet up and hold it just above its resting position over the flange. Align the slots in the toilet base with the mounting bolts, and have your helper guide the bowl so the slots fit over the bolts. Set the toilet down on the new wax ring, then sit on the toilet to apply even downward pressure, nestling the toilet onto the ring to form a seal. Install the nuts on the bolts, then snug them securely, but don't overtighten them. Install the mounting bolt caps.
Reconnect the water supply hose to the toilet tank, and tighten it securely, but do not overtighten as it can damage the gasket or break the fitting on the tank. Turn the water on and check for leaks at the supply fitting.
Allow the toilet tank to fill, then flush the toilet several times and check for leaks at the toilet base