Begin by completely clearing out the cabinet underneath the kitchen sink to create a clean and accessible workspace. Place a bucket directly under the faucet's plumbing connections to catch any residual water. Lay down several old towels to absorb any spills. Locate the two water shutoff valves on the wall or floor of the cabinet—one for hot water and one for cold. Turn the handles clockwise until they are fully closed. Then, go to the sink and turn both faucet handles to the 'on' position to release any remaining water and pressure in the lines. A headlamp is highly recommended to illuminate the dark, cramped workspace.

With the bucket in position, use an adjustable wrench or basin wrench to disconnect the two water supply lines from the threaded shanks of the old faucet. The hot line is typically on the left and the cold on the right. Turn the nuts counter-clockwise to loosen. Be prepared for a small amount of water to drain from the lines into the bucket. Next, locate the hose for the side sprayer connected to the underside of the main faucet body. This may have a threaded fitting or a clip-on connector. Unscrew or unclip it to disconnect the sprayer hose.

Identify the large mounting nuts that secure the faucet handles and spout to the underside of the sink. They are often made of brass or plastic. Use a basin wrench to grip these nuts and turn them counter-clockwise to loosen and remove them. These can be very difficult to remove if they are old and corroded. If a nut is stuck, spray it with penetrating oil and let it sit for 15-20 minutes before trying again. Once the nuts and any accompanying washers are removed, the faucet is free.

From above the sink, carefully lift the entire old faucet assembly—handles, spout, and sprayer—out of the sink holes. Set it aside. You will likely find a ring of old, hardened plumber's putty or a deteriorated gasket. Use a plastic putty knife to scrape away the bulk of this old sealant. Then, use an all-purpose cleaner and some rags or a non-abrasive scrub pad to thoroughly clean the sink surface where the new faucet will sit. Ensure the area is completely clean and dry.

Unpack your new faucet kit. Following the manufacturer's instructions, prepare the faucet for installation. This usually involves placing the included plastic or rubber gasket onto the base of the faucet spout and handles. If no gasket is provided and the instructions call for it, apply a 1/4-inch thick rope of plumber's putty to the underside of the faucet base pieces. Carefully feed the faucet's flexible lines and threaded shanks down through the appropriate holes in the sink deck, ensuring the gasket is seated properly and makes even contact with the sink surface.

From underneath the sink, slide the provided friction washers (if any) and then the mounting nuts onto the faucet's threaded shanks. Hand-tighten the nuts until they are snug. Before the final tightening, have someone look at the faucet from above to ensure it is perfectly aligned and centered. Once aligned, use the basin wrench (or a special socket tool that often comes with new faucets) to tighten the mounting nuts. Tighten them firmly to secure the faucet, but do not overtighten, as this can damage the faucet or the sink.

Take the new side sprayer and feed its hose down through the fourth hole in the sink deck. From underneath the sink, pull the hose down and connect it to the designated sprayer port on the main faucet body. Most modern faucets use a simple quick-connect fitting that clicks into place. Give it a gentle tug to ensure it's secure. If your faucet kit includes a counterweight, clip it onto the sprayer hose at the location indicated in the instructions. This weight helps the sprayer head retract smoothly back into its holder.

Connect the new (or existing) flexible water supply lines to the hot and cold water inlets on the new faucet. The inlets are usually clearly marked with red for hot and blue for cold. Thread the nuts on by hand to avoid cross-threading. Once hand-tight, use an adjustable wrench to give them an additional quarter- to half-turn. Do not overtighten, as the rubber seals inside the connectors can be damaged. Now, connect the other ends of the supply lines to the corresponding hot and cold shutoff valves on the wall, tightening them in the same manner.

Double-check that all connections are tight. Place a dry paper towel under each connection point to make leaks easier to spot. Turn both shutoff valves back on by rotating them counter-clockwise slowly. Do not open them abruptly, as the sudden pressure can damage components. With the water on, use your flashlight to meticulously inspect every connection you made: where the supply lines meet the faucet, where they meet the shutoff valves, and the sprayer hose connection. Look and feel for any drips of water.

With no leaks detected under the sink, turn the new faucet handles on, one at a time. Let the water run for a minute to flush any air or debris from the lines. Test the sprayer function. While the water is running, perform one final leak check under the sink. If everything is dry, the installation is complete. Wipe up any water in the cabinet, remove your tools and bucket, and replace the items you stored under the sink.