Put a bucket or towel under the sink
Check the stopper type
Open the cabinet under the sink
Locate the pivot rod connection
Loosen the retaining nut
Pull out the pivot rod
Lift the stopper straight up and out
Remove any clip or linkage if present
Twist the stopper counterclockwise if it is threaded
Unscrew the stopper from the drain if needed
Clean any debris from the drain opening
Reassemble the parts if required