Step 3 : Install the strainer
A strainer is a cup-like metal basket that fits into the drain opening. Some have a large lock nut that's threaded onto the strainer bowl; others that are designed to hold garbage disposers have a flange that locks on to the base of the strainer and is held in place by screws. Separate all of the parts of the strainer assembly, so you can identify them.
Apply a 1/2-inch diameter roll of plumber's putty or a bead of silicone caulk to the underside of the flat portion of the top (called the strainer flange). From inside the sink, press the strainer into the sink opening. From the underside of the sink, press the rubber gasket onto the strainer followed by the paper gasket or friction gasket. Thread the lock nut onto the strainer and tighten with slip-joint pliers.
Trim away the excess putty or caulk by running the edge of a screwdriver blade around the strainer flange. Tighten the nut a bit more until there is about 1/32 inch of putty between the sink and the strainer flange.
Attach the flanged tailpiece (the first pipe section connected to the bottom of the strainer). The tailpiece usually comes with the strainer and has a nylon insert washer the goes between the tailpiece and the bottom of the strainer.
Install the garbage disposer drain assembly by following the manufacturer's instructions, but don't install the disposer until after the sink is installed.