The unit is connected right to the bottom of the sink via a special piece that fits into the top of the sink drain mouth and extends through the drain hole. This piece is first covered with plumber's putty, which you roll in your hands into a long strand and then lay around the edge of the drain piece. Then the piece is pressed into place in the sink drain.
Most all disposals have a similar coupler unit. It is fit over the drain piece on the underside of the sink after the drain piece is installed. A wire clip is fit around the drain piece, and this holds the coupler to the unit. Then the rest of the coupler is connected by screws to the piece so it mounts tightly to the sink bottom. The actual garbage disposal unit then attaches to this coupler. It is fit into place, turned and locked. Once secured you can then connect the drain pipes.
A garbage disposal connects directly to your drain pipes above the "u" pipe neck. A PVC disposal coupler is inserted onto the drain pipe and then the pipe is fitted right to the garbage disposal. The coupler is tightened and the unit is ready to go. When installing your garbage disposal you might find that the drain piece does not fit correctly. If this is the case you need to buy another pipe neck that fits in its place. A longer pipe neck will work with larger units.