This is a starting point. Adjust to your taste.
The slow steeping method: For 8 ounces (225 g) of water, use 0.75 ounce (3 Tbsp) (28 g) of coarsely ground coffee.
Grind coffee, put in jar. Add room temperature water. Stir gently, cover and let sit 18 hours at room temperature. Filter the brew (e.g., use a paper coffee filter & pour-over dripper) into a second container.  Refrigerate. When serving, add water to dilute, milk product, and/or sweetener to taste, if desired.
The two minute AeroPress method: For an 8 ounce (225 g) cold brew, use 0.64 ounce (18 g) of finely ground coffee.
Grind coffee, put in AeroPress. Add room temperature water to #2. Stir gently for 1 minute. Gently press. Add cold water or milk product to make an 8 oz. cold brew beverage.