Roll a clump of plumber's putty in your hands into a long, cylindrical shape.
Wrap the plumber's putty around the sink flange, directly underneath the lip of the flange. The sink flange is the gasket that fits inside the rim of the drain hole at the bottom of the sink.
Slide the sink flange into the drain hole, pressing down on it.
Go underneath the sink and connect the upper mounting assembly to to the bottom of the sink flange, following the manufacturer's instructions for installing it. Either have an assistant press down on the sink gasket from above, or place a heavy object on the gasket, to press it down firmly against the sink and help provide a tight seal.
Pull away any plumber's putty that seeped up from around the sink flange.
Slide the snap ring onto the bottom of the flange underneath the sink. Pop it into the groove on the bottom of the flange and attach the mounting screws.
Knock out the plug in the dishwasher connection on the garbage disposal by holding a screwdriver against it and tapping it with a hammer. Retrieve the plug from inside the garbage disposal. (Don't do this step if you don't have a dishwasher.)
Connect the discharge tube to the discharge hole located on the side of the garbage disposal.
Place the garbage disposal underneath the mounting assembly and slide the top of the disposal into the mounting assembly. Twist the lower mounting ring until the disposal is locked into place, and tighten the ring with the provided wrench.
Connect the electrical cable to the disposal, following the manufacturer's instructions.
Connect the discharge tube to the drain trap. The drain trap is the curved part of the drain that collects water and prevents sewer gas from being vented into your home.
Restore the power to the circuit that the garbage disposal is connected to.