You need a jar, a sieve and patience. Coarse-grind 100 g of coffee, add 800 g of cold water, stir once so nothing sits dry, and leave it on the counter for sixteen hours.
Strain it through a paper filter into a clean bottle. What comes out is a concentrate: dilute one part coffee to one or two parts water or milk, over ice.
It keeps for a week in the fridge and gets rounder after two days. Our Cold Brew Blend is roasted a shade darker for exactly this, but any coffee you like hot will work cold.