Turn off the power for the kitchen at the breaker box. Label the breaker so that someone does not accidentally turn it on while you are working. Remove the existing drain from the sink by removing the retaining ring from the bottom of the sink bowl, inside the cabinet. Install the disposal drain kit, raplacing the strainer basket type with the open disposal drain. Use plumber's putty to seal the seam at the drain before you place it into the sink bowl. Twist on the retaining ring and tighten it hand tight.
Connect the electrical wire to the circuit and protect the splices using wire nuts. Red or black wires are the hot wires. White or gray wires are neutral, and green or bare wires are ground wires. Install the power switch near the sink and connect the electrical wire to it using a screwdriver to tighten the wire post on the power switch. If a dishwasher is in use, pop the drainplug out for the dishwasher drain hose using the screwdriver.
Connect the electrical wires to the disposal unit. A disposal has a three-wire system. White or gray is neutral, black or red is the hot wire, and the green or bare wire is the ground. Connect the wires according to color and protect the connections using wire nuts. Insert the wires into the disposal and replace the cover using a screwdriver.
Lift the disposal unit in place at the sink drain. Connect the disposal unit to the drain and turn the locking ring using a screwdriver to lock it into place. Remove the existing S-trap from the sink drainpipes using a pipe wrench. Wrap the drainpipe threads with plumber's tape. Attach the P-trap from the disposal drain kit and tighten the fitting with a pipe wrench. Connect the drainpipe to the disposal unit and tighten the screws with a screwdriver.
Connect the dishwasher drain hose to the disposal nipple. Secure the hose clamp using a pair of pliers. Turn the power back on to the kitchen at the breaker box. Turn the power switch on to the disposal to test it.