Assuming your fan page already exists. Its relatively easy. There is a little gear symbol in the top left of the facebook page. As you become an admin for a fan page, ect you will get an option there to use facebook as. Once you pick someone other than yourself (musical fan page, other types of fanpages, ect) you can post on their site. These types of sites don't have friends though like you do. They have likes, and most of their content is public. Let me know if this answered your question. If not I'm sure we are close.