Turn off the power to the circuit you will be connecting the garbage disposal to at the breaker box. Normally all of the outlet wiring in the kitchen will be on a single circuit. Tag the breaker so that someone does not turn it on while you are working. Disconnect the drain trap from the sink drain using a small pipe wrench to loosen the fitting.
Remove the retaining ring and gasket from the existing drain using a pipe wrench. Remove the drain from the sink bowl. Spread some plumber's putty around the bottom of the ring on the new disposal drain. Place the drain into the sink bowl. Install the gasket and retaining ring on the drain and tighten it with a pipe wrench.
Install the electrical wire and on/off switch for the disposal. Connect the wires to the disposal by inserting the end through the wire clamp on the disposal unit and twisting them together using a wire nut. Black or red is the hot wire, gray or white is neutral and the green wire is the ground. Sometimes the ground wire may be bare.
Tighten the wire clamp using a screwdriver to keep the wire from pulling out of the disposal unit. Install the cover on the disposal wire box and tighten the screws using a screwdriver. Raise the disposal up to the disposal drain. Insert a screwdriver into one of the loops on the locking ring and turn it to lock the disposal into place.
Turn the disposal so the disposal drain lines up with the drainpipe. Install the long end of the P-trap into the disposal and secure it by tightening the screws using a screwdriver. Connect the P-trap to the drain line using a pipe wrench to tighten the fitting. Do not over-tighten the fitting or you will damage the threads.
Turn the power back on for the kitchen circuit. Run water into the sink and turn the disposal switch on to test the unit.