Begin by completely clearing the cabinet area under the sink to provide ample room to work. Place a small bucket directly under the water supply connections to catch any residual water. Lay down several old towels or rags around the bucket to absorb any splashes. Locate the two water shutoff valves on the wall or floor under the sink—one for hot water and one for cold. Firmly grasp each handle and turn it clockwise (righty-tighty) until it stops. This will shut off the water supply to the faucet. Next, go to the faucet above the sink and turn the handle to the 'on' position in the middle (warm setting). This will release any trapped water pressure and drain the remaining water from the faucet and supply lines into the sink. Let it run until it's just a drip, then turn the faucet off.

Identify the leaking supply line. Since the assumed leak is at the top, start by disconnecting the bottom connection first as it's typically easier to access. Position your adjustable wrench onto the compression nut connecting the supply line to the shutoff valve. Turn the nut counter-clockwise (lefty-loosey) to loosen it. Once it's loose, you should be able to unscrew it the rest of the way by hand. Be prepared for a small amount of water to drain into your bucket. Next, address the top connection at the faucet tailpiece. This is often in a tight, awkward spot. A basin wrench is the ideal tool for this job, as it's specifically designed for these hard-to-reach nuts. Latch the basin wrench's jaw onto the nut and turn counter-clockwise to loosen. If you don't have a basin wrench, you may be able to use an adjustable wrench or channel-lock pliers, but access will be difficult. Once loosened, unscrew the nut completely and remove the old supply line.

With the old supply line removed, take a moment to inspect the threaded connection points. Use a clean, dry rag to wipe the male threads on both the shutoff valve and the faucet tailpiece. Look closely for any signs of damage such as cross-threading, deep scratches, or significant corrosion. Also, make sure there is no old plumber's tape, putty, or pieces of a broken rubber gasket remaining on the threads. A clean and smooth surface is essential for the new supply line's gasket to create a watertight seal. If you see heavy mineral buildup, you can use a small wire brush to gently clean the threads, being careful not to damage them.

Take your new faucet supply line. It's usually best to attach the top connection first, as it's the most difficult. Carefully thread the nut onto the faucet tailpiece by hand. Ensure it is not cross-threaded by turning it backwards slightly until you feel the threads engage, then turn it clockwise. Hand-tighten until it is snug. Next, connect the other end to the shutoff valve, again, threading it carefully by hand until snug. Once both ends are hand-tightened, use your wrench to give each nut a final quarter to half turn. For the top connection, use the basin wrench or adjustable wrench. For the bottom, use the adjustable wrench. Be very careful not to over-tighten, as this can compress the internal gasket too much and cause it to fail.

Before turning the water on, double-check that the faucet handle is in the 'off' position. Now, very slowly turn the handle of the shutoff valve counter-clockwise to gradually restore the water supply. Turning it on slowly prevents a sudden 'water hammer' shock to the pipes. As the line fills with water, carefully watch both the top and bottom connection points for any signs of dripping. Use a completely dry paper towel or your finger to check around each nut for moisture. If everything is dry, open the shutoff valve the rest of the way. Turn the faucet on and let the water run for a minute to flush out any air. Turn it off and perform one final inspection of the connections to ensure they are still completely dry. If you detect a small drip, turn the water off and tighten the leaking nut a very small amount (about 1/8 of a turn) and test again. Clean up your tools and remove the bucket and towels.