How do you convert a girl to Hinduism?

You can't really. One must be born a Hindu - there is no such thing as "conversion." The traditional view, would hold a foreigner as having no caste (an "untouchable" for all intents), white or whatever, and would not be a Hindu even if married to one. Thus, if you went to India, chances are you wouldn't be allowed in many temples by the priests.

Of course, you have a lot of American Hindus saying you can and you'll have no problem in an American Hindu temple, but trust me it would be a very different story if you visited India.