Begin by ensuring your work area is safe and prepared. Turn off the power to both the dishwasher and the garbage disposal at your home's main electrical panel (circuit breaker). Clear all items from the cabinet under the sink to give yourself ample room to work. Lay down old towels or place a shallow pan inside the cabinet to catch any water that will spill from the drain lines. Unpack your new dishwasher air gap kit and verify that all parts, including the body, decorative cover, washers, mounting nut, and hose clamps, are present.

Select a suitable location for the air gap on your countertop, typically near the sink basin and behind the faucet. The location must be accessible from underneath. Use a tape measure and a pencil to mark the exact center of the hole. For a laminate countertop, apply masking tape over the marked spot to prevent the surface layer from chipping. Using a power drill equipped with the correct size hole saw (typically 1-1/4 to 1-3/8 inches, check your air gap's instructions), drill a pilot hole first if your hole saw doesn't have a pilot bit. Then, drill the main hole. Apply steady, moderate pressure and let the saw do the work. Once through, clean up all sawdust from the top and bottom of the countertop.

From the top of the countertop, insert the threaded body of the air gap through the hole you just drilled. From underneath the sink, slide the large rubber washer onto the threaded body first, ensuring it sits flush against the underside of the countertop to create a seal. Follow this with the friction washer (if included) and then the mounting nut. Hand-tighten the nut as much as possible. Position the air gap body so the smaller inlet nipple faces the dishwasher and the larger outlet nipple faces the garbage disposal. Use channel-lock pliers or a basin wrench to give the nut a final quarter-turn to secure it, being careful not to overtighten and crack the plastic body.

Locate the drain hose coming from your dishwasher. It is likely connected directly to the garbage disposal's inlet or looped high up under the sink and connected to a drain tailpiece. Place your bucket underneath this connection. Using a screwdriver or nut driver, loosen the hose clamp and disconnect the hose. Be prepared for water to drain out. Route this hose (typically 5/8-inch diameter) to the smaller of the two nipples on the newly installed air gap body. Slide a hose clamp over the hose, push the hose firmly onto the nipple, and tighten the clamp securely.

This step connects the air gap to the garbage disposal. First, you must ensure the disposal's inlet port is open. If it has never been used, it is sealed by a plastic 'knockout plug' from the factory. Insert the tip of a flathead screwdriver into the inlet nipple and strike the end of the screwdriver firmly with a hammer to break the plug free. It will fall into the disposal's grinding chamber. Reach into the disposal (ENSURE POWER IS OFF) and retrieve the plastic piece. Now, take the larger 7/8-inch drain hose (usually included with the kit), attach one end to the larger outlet nipple on the air gap, and secure it with a hose clamp. Route the other end of this hose to the now-open inlet nipple on the garbage disposal. Secure this end with another hose clamp.

With all hoses connected, perform a final check. Ensure all hose clamps are tight and there are no visible kinks in either hose. Place the decorative cover over the air gap body on the countertop. Go to the electrical panel and restore power to the dishwasher and garbage disposal. Turn on the cold water at the sink and run the garbage disposal for a few seconds to ensure it is clear and functioning. Now, start the dishwasher on a 'rinse only' or 'drain' cycle. As the dishwasher pumps out water, use a flashlight to carefully inspect all your connections under the sink (at the air gap and at the disposal) for any signs of leaks. It is normal to hear some gurgling sounds from the air gap, but no water should spill out of the top vent holes.