Begin by turning off the water supply to the toilet. Locate the water shut-off valve, which is typically a small oval-shaped handle on the wall behind or beside the toilet, connected to a flexible supply line. Turn the handle clockwise until it stops. Next, remove the tank lid and place it carefully on a towel or rug to prevent it from cracking. Flush the toilet by pressing and holding the flush handle down. This will drain almost all of the water from the tank, making the flapper accessible and preventing a mess. Use a sponge or an old towel to soak up the small amount of water remaining at the bottom of the tank.

With the tank empty, locate the old flapper. It's the rubber or plastic dome covering the large opening at the bottom of the tank (the flush valve). First, unhook the chain connecting the flapper to the long flush lever arm. Then, carefully detach the flapper itself. Most flappers have two flexible 'ears' or rings that slide over two small pegs on the vertical overflow pipe. Gently bend the ears and slide them off the pegs. Lift the old, worn flapper out of the tank and set it aside.

Before installing the new flapper, it is crucial to clean the surface where it will sit. This surface is called the flush valve seat, the smooth, circular rim of the drain opening. Over time, this seat can accumulate mineral deposits, sediment, or bits of deteriorated rubber, which can prevent a perfect seal. Use a cloth or a non-abrasive scrub pad to wipe the rim thoroughly until it is completely smooth and clean to the touch. A clean seat is essential for preventing leaks.

Take your new toilet flapper and position it over the flush valve opening. Align the two ears or rings on the flapper with the corresponding pegs on the sides of the overflow pipe. Gently stretch the ears and slide them down over the pegs, ensuring they are securely attached. Double-check that the flapper is centered over the opening and can move up and down freely without catching on anything.

Connect the clip on the new flapper's chain to one of the holes on the flush lever arm (the rod connected to the outside handle). The chain's length is critical. Adjust it so there is just a small amount of slack, about 1/2 inch, when the flapper is closed. If the chain is too tight, it will prevent the flapper from seating properly, causing a leak. If it's too loose, the flapper won't lift high enough for a full flush, or the excess chain could get caught under the flapper. Once the length is correct, you can use scissors or wire cutters to trim any significant excess chain.

Slowly turn the water shut-off valve counter-clockwise to restore the water supply to the toilet. The tank will begin to fill. Watch the new flapper as the water rises to ensure it's seated correctly and no water is visibly leaking from the seal. Once the tank is full, listen carefully. You should not hear any water running. Now, perform a test flush. The toilet should flush with full power, and the flapper should drop back down and create a tight seal once the water has drained from the tank. After the tank refills again, wait a minute and listen to confirm the running has stopped. If all is quiet and the flush is strong, the replacement is complete. Carefully place the tank lid back on top of the tank.