Pour directly down the drain. If the baking soda clumps, use a funnel or a spoon to push it down. Ensure as much powder as possible makes it into the pipe.
Immediately cover the drain with the drain plug or a wet rag. You want the expanding gas and foam to be forced into the pipes to attack the clog, rather than bubbling up out of the sink. unclog drain with vinegar baking soda
If vinegar+baking soda fails, try:
Boil a full kettle of water (about 2 liters / half gallon). Carefully pour the down the drain. The hot water helps flush out loosened debris. Pour directly down the drain