Take 8-10 oz of your ground coffee of choice and put it into a plastic container. Add about 24 oz of filtered or drinking water and cover. Let it sit at room temp for a couple of days, shaking it every now and then. After a couple of days, pour the entire contents into a French press to extract the coffee or if you don't have a French press, rig up a coffee filter setup and strain. Take 1/2 of the resulting cold brewed coffee and add it to your bottling bucket or keg prior to racking. After racking, taste and add more coffee if needed until you get the desired coffee flavor.
I did this recently on a mocha stout and went with 6 oz of course ground coffee for a 4 gallon batch. The coffee was evident in the beer, but I thought it could use a little more, which is why I suggest 8-10 oz (assuming a 5 gallon batch at bottling/kegging). And the amount of flavor that comes through all depends on the particular coffee and the grind, so that is why I suggest you add half to begin with, then add more if needed.