Let's say I am allowed contemporary pilgrims / travellers attire and start in an international port where they are used to strangers looking and acting strange. London, 1200. Claim to be a pilgrim from a mysterious Christian kingdom (Prester Johns) in Africa. I don't think they would be worried that I am too white. Try hard to remember high school Latin, latinize English words back. A guy in pilgrims clothing and having some idea of Latin and having interesting stories - or at any rate can read or write - is not a beggar, lower-middle class status like an ex-friar turned scribe, can be a middle-class family's interesting guest. Claim we are a very pious folks and be very, very religious, to earn trust. Start, for example, linking up with the traders in the port who are probably fairly open-minded. Be the guest of a merchant who is interested in info about foreign markets (make it up). See if I can teach things, like accounting they find useful. Claim the Holy Ghost taught Prester John all kinds of marvelous things he then taught us. Don't try scientific explanations, bu also beware not to look like warlock, rather try to present all the knowledge as the good kind of magic, the church kind. Pick easy elements from this list: http://www.topatoco.com/graphics/qw-cheatsheet-print-zoom.jpg and claim it was all taught by the Holy Ghost to Prester John.