Begin by ensuring the work area is safe. Go to your home's main electrical panel (breaker box) and shut off the circuit breaker that supplies power to the kitchen sink area. Place a non-contact voltage tester near the outlet to confirm the power is off. Clear everything out from the cabinet under the sink to give yourself ample room to work. Lay down old towels and place a bucket directly under the P-trap to catch water when you disconnect the drain.

Using channel-lock pliers, loosen the slip nuts on the P-trap that connect it to the sink's tailpiece and the drain arm extending from the wall. Once loose, you can usually finish unscrewing them by hand. Carefully lower the P-trap and empty any contained water into the bucket. Next, loosen the large nut holding the existing sink strainer assembly to the bottom of the sink. You may need another person to hold the strainer from above to keep it from spinning. Once the nut is removed, push the old strainer up and out of the sink drain opening. Scrape away any old plumber's putty from the sink opening.

Take a small amount of plumber's putty and roll it into a snake-like rope about 1/2-inch thick. Press this rope firmly around the underside of the new sink flange (the metal drain piece). From the top of the sink, press the flange down into the drain opening so it is seated evenly. From underneath the sink, slide the fiber gasket, then the backup flange (a metal or plastic ring), and finally the mounting ring over the sink flange's neck. Secure these by threading on the snap ring; push it on until it pops into the groove on the flange. Tighten the three mounting screws on the mounting ring evenly with a screwdriver. This will squeeze the backup flange up, compressing the putty and creating a watertight seal. Scrape away any excess putty that squeezes out in the sink basin.

Before mounting the disposal, you must prepare it. If you are connecting a dishwasher, lay the disposal on its side and locate the dishwasher drain inlet. You will see a knockout plug inside the inlet. Insert the tip of a screwdriver into the inlet and tap it firmly with a hammer to knock the plastic plug out. Reach into the grinding chamber and retrieve the loose plastic plug. If your disposal did not come with a power cord pre-attached, you will need to install one now by following the manufacturer's specific wiring instructions.

This is often the most physically challenging step. Lift the garbage disposal up to the mounting ring. Align the three tabs on the top of the disposal with the ramped grooves on the lower mounting ring. Once aligned, turn the lower mounting ring clockwise until the disposal tabs lock securely into place. You will feel and hear it lock. Most disposals come with a small 'wrenchette' or you can use the end of a screwdriver in the lugs of the mounting ring to get the leverage needed for the final turn.

First, attach the plastic discharge tube (usually an L-shaped pipe) to the side of the disposal using the supplied gasket and flange/screws. Hand-tighten the screws, then give them a final quarter-turn with a screwdriver. Next, connect the dishwasher drain hose to the dishwasher inlet you prepared earlier and secure it with a hose clamp. Finally, connect the disposal's discharge tube to the P-trap. You may need to cut a section of straight PVC pipe with a hacksaw to get the alignment right. Dry-fit all the P-trap pieces first, then secure all slip nuts. Hand-tighten them, then give a final quarter-turn with channel-lock pliers.

Plug the disposal's power cord firmly into the switched electrical outlet under the sink. Go back to the electrical panel and turn the circuit breaker back on. Place the sink stopper in the sink drain and fill the sink basin about halfway with cold water. Now, for the moment of truth: turn on the wall switch for the disposal, then pull the sink stopper out. Let the water and disposal run while you carefully inspect every connection point for leaks with a flashlight: the sink flange, the discharge tube connection to the disposal, all P-trap slip nuts, and the dishwasher hose connection. If you see any drips, turn everything off and tighten the leaking connection slightly.