Mix the coffee and water in a large covered jar or pitcher.

After 4 hours or so, the coffee grounds will separate and float to the top. Stir to remix them.

Cover the jar/pitcher and leave at room temperature overnight.

Strain into another jar/pitcher through triple fold of muslin. Dump the grounds in the muslin and give that a good squeeze.

Refrigerate.

Serve using a 1 to 2 ratio (one third coffee concentrate to two thirds cold water).

I would also suggest mixing up some and freezing it into ice cubes, so you don't water down your beverage.

It will keep at least a week.