In a large jar or pitcher, add the ground coffee.
Pour in the cold filtered water, making sure all the grounds are fully saturated. Stir gently with a long spoon to combine.
Cover the jar or pitcher with a lid or plastic wrap, and place it in the refrigerator for at least 12 hours, or up to 18 hours for a stronger brew.
After steeping, strain the coffee through a fine-mesh strainer lined with a coffee filter or a piece of cheesecloth into another clean pitcher or jar to remove the grounds. Discard the grounds.
To serve, fill a glass with ice cubes, then pour in the cold coffee concentrate. If desired, add whole milk or your favorite creamer, and sweeten with granulated sugar or simple syrup to taste.