Turn off the water supply
Place a bucket under the sink
Remove the old drain pipe if needed
Clean the sink drain opening
Install the sink strainer with plumber’s putty or gasket
Tighten the strainer locknut from below
Attach the tailpiece to the strainer
Connect the P-trap to the tailpiece
Align the trap arm with the wall drain
Tighten all slip nuts by hand
Use slip-joint pliers if needed for a snug fit
Check that all washers are seated properly
Run water to test for leaks
Tighten any leaking connections
Recheck the drain alignment and slope
Dispose of old parts and clean the area