Summarize the following:
Decide which kind of drain cleaner that you want to use. Options include traditional cleaners, foaming cleansers, and enzyme cleaners. Traditional and foaming cleansers may contain chemicals that are harmful to the environment while enzyme cleansers are made of natural ingredients. Because all drain cleaners are different, it's important that you read the instructions so that you know how long you should let the drain cleaner sit before flushing it.  Enzyme cleaners work best in warm water rather than hot water. Do not combine two different types of chemical drain cleaners or it can create toxic fumes. Pour the appropriate amount of drain cleaner down the sink according to the directions. If your drain is clogged, you'll see the solution settle near the top of your drain pipe. After letting the drain cleaner sit in the drain, you will have to flush the rest of the cleaner down the drain with warm or hot water.  Follow the directions on the packaging for the optimal results.

Summary:
Purchase a drain cleaner from the store or online. Read the instructions on the packaging. Pour the drain cleaner down the drain. Flush the drain with hot or boiling water.