While Lyra uses the alethiometer as a means of divination, Mary Mallone uses the method of I Ching which currently exists. The method Mary chooses to use is by using yarrow stalks arranged on a cloth of silk. She then divides the stalks in a zen like state and interprets the shapes left by the pictures of trigrams in the book.
For a little bit more information, Wikipedia holds some answers.
Or you could read the Book of Changes (aka I Ching) here and explore for yourself.
If you don't fancy that much reading and you're lucky enough to have an iPhone, then lo and behold there is quite literally an app for everything. So here is the I Ching app. Otherwise there are some other great resources here.