We use the Illy coffee pods that work in most espresso machines (well, the Waitrose own-brand version); they're like a fat round tea bag and they simplify cleaning up. But we have a handful of hotel coffee single-brew, sealed in a filter bag coffee pouches that make terrible coffee in a cafetiere, and there's always a dog-end of ground coffee hanging around in the back of the fridge, so I thought I'd try this out.
Coarse ground coffee is recommended in most of the recipes, and the proportion is 4:1 water to coffee - so a third of a cup of coffee to a cup and a half of water. Use cold water, in a jug and mix the ground coffee if it looks as if it's forming lumps in the water, then leave for at least 3 hours and more likely overnight, especially if you're doing it in bulk.
When it looks ready, strain. I used a tea mesh ball (like a mini sieve) lined with a double folded sheet of tea bag paper that came as packing in an Republic of Tea order; muslin would also work. The woven cotton sock I use for brewing herb teas inside the sock didn't strain quite as effectively (I am comparing a brew of hotel + bagged coffee to all bagged coffee). Next time I'm going to try reusing the filter bag from the hotel coffee pouch ;-)
What you get is strong, cold coffee; a little on the strong side to drink black, ideal diluted 3:1 with milk - US recipes often dilute with cream and water, but cream in coffee is a very US thing ;-)
The cold brewing turns awful hotel coffee into something very drinkable; you get far less acidity and bitterness - either less of the coffee oil comes out or not heating it makes a difference. And there's none of the powdery sludge you get from the bottom of the pot. Straining is fiddly; doing the cold brew in a cafetiere (without plunging) and using a wide mouthed bottle to pour into make it much easier. Other than that, easy and a nice way of getting real iced coffee on tap.