Clean the sinks, and pour a kettle of boiling water into them and on their sides. Some people pour hot water mixed with bleach down the drain. Cleaning, without any other kashering procedure, is sufficient. If they were used for hot chometz they should first be cleaned well. Then either boiling hot water should be poured on them, or they should be completely covered so that nothing Pesach dik touches them.
Some people do both.
Wash them with a household cleaner. The usual practice is to cover the tables. Wash the top and side surface areas with a rag soaked in a household cleaner. Some people cover it with aluminum foil. Old grates can be kashered by first cleaning them and then lighting all the burners, raising them to their maximum heat, and preferably putting on a blech while the burners are on.