After turning off the water at the shutoff, flush the toilet to remove most of the water from the tank and bowl. A little water will be left in the bowl. Use a force-cup plunger to force it down the drain line. Remove the remaining water from the tank with a sponge and a bucket.
Disconnect the supply line from the base of the tank.
To remove the old toilet, pop off the trim caps at the base of the toilet. Next, loosen and remove the nuts and washers on the closet bolts securing the bowl to the floor. If the nuts are rusted, you may need to cut them off with a hacksaw.
Note: If you're working alone, disconnect the tank from the bowl before disconnecting and removing the bowl. Lifting the two pieces separately will be a lot easier.
Now that you've removed the nut and washer on each side of the bowl, gently rock the toilet back and forth until you can lift it free. Once you've removed the toilet, there will be an open line to the sewer system. Stuff a rag into the hole to prevent sewer gases from backing up into your home and to prevent tools from falling into the hole.
Use a putty knife to pry up and remove the old wax seal (Image 1). Inspect the flange beneath the seal to make sure it's not cracked. Once you've inspected the flange, remove the rag from the hole. If the flange is damaged, get help from a plumber. If the flange is OK, remove the rag from the hole and take the closet bolts out of the flange. Install the new wax seal, and hook the new closet bolts into their slots on the flange (Image 2). Important: Remove the rag before installing the new wax seal. Leaving it in is a common and costly mistake.
With the closet bolts and seal in place, you can attach the tank and bowl to assemble the new toilet. Slide the tank bolts through the opening in the bottom of the tank. Line up the shank of the bolts with the holes in the bowl, and drop the tank into place. Secure the tank to the bowl by tightening a nut onto each tank bolt. Evenly tighten all the nuts until the tank rests firmly on the bowl.
Set the assembled toilet onto the flange assembly, making sure that the closet bolts protruding from the floor line up with the holes in the base. Lightly press the toilet in place to form a good seal with the wax ring.
Place a couple of washers and a nut on each bolt. Secure the nuts with a small wrench, but don't over tighten them. If the bolts extend too far over the top of the washers and nuts, cut off the excess with a hacksaw.
Pop the trim caps in place to cover the ends of the bolts.
Secure the new seat and lid to the bowl with mounting bolts.
Use slip-joint pliers to reconnect the supply line to the bottom of the tank.
Keeping toilets in good repair will save you money. Even a small leak may waste as much as 40 gallons of water per day.