Turn off the water supply
Place a bucket under the sink
Remove the old drain assembly
Clean the sink drain opening
Apply plumber’s putty or silicone under the drain flange
Insert the drain flange into the sink opening
From below, attach the rubber gasket, washer, and locknut
Tighten the locknut securely
Wipe away excess putty or silicone
Connect the tailpiece to the drain
Tighten all slip nuts
Install the P-trap
Check alignment of all parts
Turn on the water supply
Test for leaks
Tighten any leaking connections if needed