In the old days Quark used to be made by letting milk sit. It would turn thick
and sour by itself. Today it is impossible because the milk has been
pasteurized and will just turn bad. If you have access to unprocessed milk,
than you can still do it the old fashioned way. But be aware, that the milk
might be contaminated by other bacteria. The safest way is to buy processed
milk and inoculate it. Inoculation is nothing else but putting the right
bacteria to the milk. You can either buy the bacteria (New England
Cheesemaking Supply Company) or start you own culture.

http://www.german-usa.com/german/recipes/hints/cheesemk.html - this site may
help you!