Mix 1:1 water and flour, some salt, leave in warmth for a while. Two or three days should be enough to start seeing bubbles. During the first week you should be taking away half of the starter, disposing of it (can try baking already, results will vary day by day), and refilling the starter with fresh flour, water and salt. Maintain constant ratios. This is to ensure your starter isn't dominated by wrong microbes.
Take some starter out every few days (you use it by mixing it with fresh flour, some water and leaving it for a few hours under cloth), and return what you took from the starter in flour and water (1:1 or experiment slightly).
 - My proportions: glass (250 cm^3) of flour, glass of water, two or three tablespoons of salt.
EDIT: Why salt? It's not something you need. Really, you can leave it out and get good sourdough. I opt for adding it because it makes the initial process shorter and a bit more forgiving. As time goes by, you can phase it out by simply not adding salt with water and flour after you take some for baking.