If you're installing a new sink, attach the disposal to the sink before you set the sink in place. Otherwise, use plenty of towels to make the undersink work space as comfortable as possible. If you've never attempted plumbing or electrical projects before, this may not be the best one to start with. But provided you have basic knowledge, it's really not difficult.
Before you start, be sure to check local codes, which may regulate the use of disposers. Codes may require that a dishwasher drain hose be routed through an air gap before entering a disposer.
Caution: To avoid electrical shock, be certain electricity is shut off at the main panel before you begin.
Note: Before starting this project, you may need to install a switched electrical receptacle. If you purchase a self-switching disposal, you can plug it into a standard, always-live receptacle.
Disconnect the sink trap and remove the basket strainer. Clean away all putty. Take apart the mounting assembly by removing the snap ring, mounting rings, and gasket from the flange. Lay a rope of plumber's putty around the sink opening. Have a helper hold the flange in place as you work from underneath. Slip the gasket, mounting rings, and snap ring up onto the flange. The snap ring keeps the mounting assembly in place temporarily. Tighten the mounting assembly against the sink by tightening each screw clockwise a little at a time to assure a tight seal. Using a putty knife, shave away excess putty.
Remove the electrical cover plate on the disposal. Strip insulation from the wires of an approved appliance cord, slip it into the opening, and clamp it. Make the electrical connections, gently push the wires into the cavity, and reinstall the cover plate. Secure the drain elbow to the disposal. If you'll drain a dishwasher through the unit, remove the knockout inside the nipple. Position the disposal and rotate it until it engages and tightens. Once the connection is made, rotate the disposal to the best position for attaching the drain lines.
Fit a slipnut and rubber washer onto the drain elbow, then fasten the trap to the elbow and the drainpipe. You may need to cut the elbow to make the connection. (For double sinks, connect the elbow to the second bowl drain.) Connect the dishwasher drain hose to the drain nipple of the disposal and fasten it with a hose clamp. If you're using a regular electrical receptacle, simply plug in the electrical cord. For a hard-wired installation, shut off the power and connect the wires as shown. Restore power and test for leaks and excessive vibration.