Allow the caulk on a top-mount sink to cure for 24 hours. Instead of making the final connections right away, it’s better to give the silicone caulk time to set up. This will prevent the sink from shifting and breaking the solid bead of caulk that you made. If you’re installing an undermount sink, you should already have waited 24 hours before putting in the sink drain. In this case, you can proceed with the last steps of the installation.Reconnect the water lines and the P-trap underneath the sink. You just need to do the reverse of the disconnecting process. Hand-tighten the water lines where they connect to the hot and cold shutoff valves or use a crescent wrench if needed. Similarly, use your hands to tighten the nut on a PVC P-trap or channel locks for a metal P-trap.If your new sink drain is a little shorter than your old one, you can buy a P-trap pipe extension at your local hardware store. The extension can be cut to fit and will also connect in place with a nut that you’ll either hand-tighten or secure with channel locks. If your new sink drain is a little too long, you can cut off some of the pipe either at the top of the P-trap or the bottom of the drain. Use a hacksaw or pipe cutter to make the adjustment.Turn the water back on and check for leaks. Open the hot and cold water valves by turning them counterclockwise. Then, open the hot and cold faucet taps fully and let the water run for at least 2-3 minutes. Watch under the cabinet for any leaks in the water lines, drain lines, or elsewhere. Tighten any loose connections as needed.Keep the bucket or towel at the bottom of the sink cabinet while you test for leaks. If you have a leak at a pipe connection, try shutting off the water, undoing the connection, wrapping some plumber’s tape around the pipe threads, and then re-making the connection. If you can’t figure out where a leak is coming from and/or how to fix it, shut off the water supply lines and call a plumber.