Assuming you are referring to the popular Chemex coffee maker, here are detailed instructions on how to clean it:
First, remove the filter and dispose of the used coffee grounds. Rinse the filter with hot water to remove any remaining grounds.
Next, pour hot water into the carafe up to the line indicated on the inside. Add a teaspoon of baking soda for every cup of water used (e.g., if you use 4 cups of water, add 4 teaspoons of baking soda). Swirl the mixture around and let it sit for a few minutes.
After a few minutes have passed, begin pouring out the hot water/baking soda mixture. Be sure to do this slowly so that all of the mixture has a chance to exit through the spout. Once all of the mixture has been poured out, rinse the carafe with hot water several times until all traces of baking soda have been removed.
Now your Chemex is clean and ready for use!