Boil water and pour it slowly down the drain
Remove and clean the drain stopper or strainer
Use a plunger to loosen the clog
Pour baking soda into the drain
Add vinegar and let it sit
Flush with hot water
Use a drain snake or auger to pull out the blockage
Remove and clean the P-trap under the sink
Check and clear the overflow opening if present
Use a wet/dry vacuum to suction out the clog
Repeat the process if needed
Call a plumber if the clog will not clear