Remove the lid from the top of the toilet tank and inspect the flapper to see if it is seated completely in the hole on the bottom of the toilet tank. Sometimes the chain connecting the flapper to the handle is tangled and will not allow the flapper to set all the way down in the hole. If this is the case simply untangle the chain and test to see if that fixed the problem. If this did not fix the problem a new flapper will need to be purchased.
Shut off the water to the toilet by turning the shut-off valve that feeds the water line going into the toilet tank. Turn this valve clockwise as far as it will go.
Flush the toilet and let the water drain out of the toilet tank. With the shut-off valve turned off, the tank will not refill.
Remove the old flapper from the bottom of the toilet tank. Usually the flapper is attached by two pegs on either side of the hole in the bottom of the tank. The other end of the flapper is attached to the handle by a chain. Pull the flapper off the pegs and remove the chain.
Install the new flapper by attaching it to the bottom of the toilet tank on the pegs and attaching the chain to the other end. If a universal flapper was purchased, there is a rubber piece between the two ends that attaches to the pegs. This will need to be cut out. Make sure the chain is long enough to set the flapper into the hole, but not too long that it may get tangled.
Turn on the shut-off valve and allow the toilet tank to fill. Flush the toilet and make sure it is working properly before replacing the lid.