Shine a flashlight underneath the sink, and locate the hot and cold water shut-off valves. The valves are normally located on the rear wall underneath the sink. Twist both the hot and cold water shut-off valves completely to the right.
Turn on both the faucet's hot and cold water handles. Lift the faucet lever straight up, in the neutral position, if the faucet is a single-lever model. This releases any remaining pressure in the water lines.
Unscrew and disconnect the nuts on both the hot and cold water supply lines, connecting the lines to the shut-off valves. Use an adjustable wrench. Unscrew and disconnect the nuts on the other end of the lines, connecting the lines to the faucet inlets, using the adjustable wrench. Disconnect the water line from the faucet inlets using a basin wrench, if you are unable to reach the nuts with the adjustable wrench. The basin wrench is specifically designed to reach these hard-to-reach connections underneath the sink.
Measure the old supply lines, using a tape measure. Measure the distance from the nipples on the shut-off valves to the nipples on the hot and cold water inlets underneath the faucet, using the tape measure. Make certain that the replacement supply lines are long enough.
Wrap plumber's tape around the nipples on both the hot and cold water shut-off valves. Wrap the tape a couple of loops around each nipple.
Connect the replacement supply lines to the shut-off valves, tightening the nuts with an adjustable wrench. Connect the other end of the supply lines to the corresponding hot or cold water inlets underneath the faucet. Tighten the nuts with the adjustable wrench, being sure not to over tighten.
Turn on the hot and cold water shut-off valves. Check for any leaks from the supply lines, from where they connect to the shut-off valves and the faucet inlets. Tighten the nuts, if there is a leak. Turn on the faucet's hot and cold water, and again check for any leaks from the supply lines. Tighten any connections, using the adjustable wrench, if necessary.