Slowly pour 1 cup of baking soda into the drain hole. Use a spatula or spoon to help guide the baking soda into the hole.
Pour 1 cup of white vinegar down the drain. Immediately plug the drain hole after pouring in the vinegar.
Let the mixture sit in the drain for 30 minutes. In the meantime, boil 4 cups of water on the stove.
Open the drain plug and slowly pour the hot water down the drain. Turn on the tap to see if the clog has cleared. If the water is still backing up, repeat this entire process. It may require more than one attempt to effectively clear the drain.