The friend who's working in Chicago decided to make home-made barley (with a little help long-distance from his mum) and he actually prepared 3 litres of it! Since I can only boil water, I'm impressed and asked for his drink recipe. Here's his reply:

ingredients:
(a) 1 cup (400ml) of barley (almost 1 pound)
(b) 3l of water in a big pot
(c) half a lemon, sliced
(d) rock sugar

1. wash (a) (supposedly, barley from china works better)
2. boil (b) with (a) thrown in. simmer after the water has boiled.
4. add (d) to taste. i put in about 100g.
5. when the barley has "opened" (it'll look fluffy and soft at this point), stop heating and add (c).

ta da! barley water ...