Combine the coffee and water in a container. Stir well, making sure the coffee grounds are all wet.
Cover and leave on the counter for at least 12 hours, up to 24.
Strain coffee through a coffee filter set in a strainer into another container. This is your cold brew concentrate, and it can be kept in the fridge for up to 2 weeks.
To make coffee, add 1/4 cup of the cold brew concentrate to a coffee mug. For regular hot coffee, add 3/4 cup of boiling water, or for an iced coffee, use cold water and serve over ice.