A new sink drain assembly, pipe wrench, plumber's tape, silicone caulk, pail, and potentially pipe cutter are needed.
Open the faucet and let it run until the sink is empty after turning off the water.
Unscrew and remove an old drain with a pipe wrench. Prepare for residual water spills; position a bucket below.
Remove old plumber's putty or sealant from sink drain hole. A good seal with the new drain requires thorough cleaning.
Put the new drain in the sink. Install the rubber gasket under the sink if the assembly has one. Prevent leaks with plumber's tape on threads.
Attach the drain flange nut from underneath the sink. Tighten it with a pipe wrench, but not too much to damage the sink.
Install the drain pipes on the new drain assembly. Use plumber's tape on threads and snug any connections to avoid leaks.
Restart the water and run the faucet to check for leaks. See any? Tighten the connections or reapply plumber's tape.