The seal, on the bottom of a toilet, where it rests on the floor, is a gasket made of a ring of wax.

This inexpensive seal can help prevent damage to flooring and sub-flooring… expensive items to repair if a leak is not repaired right away. Be sure to do this procedure when the temperature is at least 23 degrees celcius so the wax will be pliable.

If you are replacing a leaky seal on an existing toilet, you will want to wear rubber gloves and start at Part 1; if you are installing a toilet in new construction, skip to Part 2.

Part 1

Be sure to shut off the water supply.
Flush the toilet once or twice to empty the tank and bowl.
Soak up and remove any remaining water in the tank and bowl
Remove the caps over the nuts that hold the base of the bowl to the floor.
Remove the nuts. You might need to use oil and pliers.
Disconnect the water supply line.
Gently rock the toilet back and forth to break the seal and any caulking present. Use care to grip the bowl and not the tank. If you grab the tank, you could break or crack the tank where it is bolted to the bowl.
Remove the remnants of the old seal from both mating surfaces. Go to Part 2.

Part 2
Uncover the new wax seal, and place it over the flange on the floor with the round side up, exactly centered over the opening. Be sure both bolts are in place.
Lift the toilet and place it over the flange and seal, keeping the toilet as level as possible.
Sit on the toilet to help the new seal do its actual sealing.
MAke sure the bowl is level.
Replace the nuts removed earlier. Tighten the nuts with your hand to avoid cracking the base of the bowl.
Reconnect water line.
Turn on water supply to let tank fill. You can turn the water supply on part way to let the tank fill with lower pressure. This will take a bit longer, but the part-way setting helps prevent the dreaded “Who just flushed?” scream from another household member that may be scalded in the shower because of a surge in the cold water going into the toilet tank.
Flush and check for leaks.
Apply silicone gel as needed to junction of bowl and floor.
Wash hands with an anti-bacterial soap.