Remove the toilet tank lid and set it out of the way.
Turn the shutoff valve on the floor next to the toilet all the way to the right to stop the water supply to the toilet. Flush the toilet to remove most of the water in the tank.
Search for the float cup valve, which is on the left side of the toilet. Notice how the plastic cup is attached to the tower.
Apply plumber's grease with a cotton swab to the tower and the inside of the cup where the tower inserts into it. Move the cup up and down the tower to work in the grease.
Turn on the shutoff valve to supply water to the tank. Flush the toilet several times to ensure that the cup is working properly. Replace the tank lid.