1/2cup ground coffee (medium-coarse grind is best)
In a jar, stir together coffee and 1 1/2 cups cold water. Cover and let rest at room temperature overnight or 12 hours.
Strain twice through a coffee filter, a fine-mesh sieve or a sieve lined with cheesecloth. In a tall glass filled with ice, mix equal parts coffee concentrate and water, or to taste. If desired, add milk.