Invitation Letter for Visa. Traveling to Sichuan/Chengdu with a friend who is a Chinese Citizen.

Hello. I am traveling to Sichuan (Chengdu) with my boyfriend who is a Chinese Citizen ( I am a citizen of the U.S.). I read about the invitation letter needed for my visa. We are planning on a 2-3 week visit in mid-March to see his family and travel a bit within China. What exactly should his parents (whom I will be staying with part of the time) write in the letter, and who is the letter addressed to? I know they need my name, gender, and passport number, but what exactly is needed from them? Can they fax the letter to me or submit it to their Embassy in Sichuan. Thank you for your help!