If there is no standing water in the sink, add 2 to 3 inches of tap water.
Seal all air vents and adjoining drains with duct tap or rags in order to create a vacuum. This is necessary so that the water in the drain forces the clog down the pipe.
Cover the drain completely with the suction cup portion of the plunger.
Use both hands to push and then pull the plunger handle up and down a few times. It is just as important to pull as it is to push. Use as much force as possible. The idea is to get the clog to move through the pipe in one direction or the other.
Remove the plunger, and wait a few seconds to see if the water drains.
If the water does not drain, repeat the process until the clog breaks and moves, allowing the water in the sink to drain.