Quickly cover the drain opening with a stopper or damp cloth to force the reaction downward into the pipe, not upward. Leave for 5–15 minutes (longer for tougher buildup).
Immediately pour 1 cup of white vinegar into the drain. A vigorous fizzing reaction will begin. how to clean shower drain with vinegar and baking soda
Pour a kettle of boiling water directly down the drain to soften and loosen surface grease. Quickly cover the drain opening with a stopper