Close the shutoff valve leading to the toilet tank.
Flush the toilet and remove the toilet tank's lid.
Unhook the steel chain from the flapper which runs down from the flush handle's arm.
Unhook the flapper from the two tabs on either side of the flush valve. These tabs let the flapper pivot up and down in place.
Slide the old flapper up and over the top of the flush valve.
Cut your new universal flapper with a pair of scissors to match your old flapper.
Slide the round opening of the new flapper over the top of the flush valve all the way to the bottom of the toilet tank and hook the new flapper back onto the tabs on either side of the flush valve.
Reattach the steel chain to the rubber loop on top of the universal flapper.
Open your shutoff valve, letting the tank fill back up with water and watch to see if water continues to run into the toilet bowl.