This easy process creates a base that can be diluted with water or milk for your favorite cold coffee drinks. Filtered or bottled water may be used, but if your tap water tastes good, go ahead and use it. That's what I do.
Add the ground coffee and cold water to a large bowl or pitcher. Stir to combine. Cover, and allow the mixture to sit at room temperature for at least 12 hours but no more than 24 hours. You may refrigerate, if preferred.
Set a fine mesh strainer over a large bowl, and place a coffee filter or several layers of cheesecloth in the strainer (see notes). Slowly, pour the coffee mixture into the lined strainer. The coffee will take some time to drip through. If using a coffee filter, you will have to pour in batches so as not to overflow the filter.
Once all of the coffee has filtered through the strainer, discard the grounds (or add to a compost pile). Store the coffee base, covered, in the refrigerator for up to 1 week. To serve, pour the cold coffee base over ice and dilute with water or milk as desired. A one-to-one ratio is a good starting point; feel free to adjust and/or add sweetener of choice.
Notes
Cheesecloth may allow a little bit of coffee sediment to slip through, but the end result will still be good. One friend told me that she has used an old--clean!--knee high nylon as a strainer. A very thin old tea towel or even sturdy paper towels could be used in a pinch. If you have a drip coffee machine, you may place a filter in the basket and strain the coffee directly into the coffee pot below.