Most clogs in the average household drain pipe fall into one of two classifications -- grease (fats) or hair (protein). The method that will work best depends on which type of clog you have. With grease clogs, the most important task is to liquefy the solidified grease. Hair clogs are usually made of tangled clumps of hair held together by trapped soap scum. The key to removing stubborn hair clogs is to dissolve either the hair or the soap scum that keeps the hair clog together.
Make sure the sink has no standing water. Standing water will prevent the hot water from making contact with the grease clog. If there is standing water, scoop out as much of it as possible. If you have an old turkey baster or injector, try sucking any standing water out of the drain.
Pour the boiling water directly down the drain until the water backs up into the sink. The heat from the water will start softening and liquifying the grease.
Watch the water level to see if water starts flowing down the drain. As the water drains, keep adding boiling water until water starts flowing steadily down the drain.
Pour 1 cup of undiluted bleach down the drain. The bleach will dissolve the strands of hair breaking up the clog.
Let the bleach sit in the drain for 1 hour. Do not run water into the sink during this time. Doing so will dilute the bleach and make it less effective.
Run hot water into the sink and observe the flow of water down the drain. If the flow is still sluggish, allow the water to drain then repeat the process.