Unscrew the diverter valve attached to your faucet. You should be able to unscrew it by hand, but if it is on too tight, use pliers.
Look inside the diverter valve, and pull out the existing gasket with needle-nose pliers.
Insert the new gasket into the diverter valve, and push it into place. You may use the needle-nose pliers to help push it into place if you cannot do it by hand.
Replace the diverter valve by screwing it back into place on the faucet.