First make sure water is drained from tub or sink. If it is slow draining, just be patient and wait for all the water to drain.Remove any drain plugs or stoppers from the tub or sink. You want an opening big enough so you can really get the solution down the drain.
Next pour boiling water down the drain to help loosen up any gunk. Let water drain again.
Next pour baking soda down drain. This was by far the hardest and most time consuming part. Try your hardest to get all of the baking soda down there. Use a funnel if you have to!
Next slowly pour in the lemon juice, and also vinegar works just as good! The lemon juice will cause a reaction with the baking soda and this will result in some fizzing and bubbling action. Cover the drain with a cloth, or tub stopper so that the baking soda can work. We don't want it to come up, we want it to break up the clog down below.
Leave the cloth or drain plug for 30 minutes. During this time the foaming action will be breaking down the clog.
After 30 minutes, uncover the drain and very slowly pour more boiling water down to flush the clog out. If you have plastic piping, it is best to avoid boiling water and instead use very hot water.
Depending on how stubborn of a clog you have, you may need to repeat the process a few times, but it should work if you follow these steps. If you're still having troubles it may be time to humble yourself and call the plumber. :)