AskFantasyHistorians flair is only available for our regular AskHistorians flaired users who have gone through our vetting process as an expert in a particular historical area.
Basically give this post a read, and if you've got the right stuff, submit your application right there
http://www.reddit.com/r/AskHistorians/comments/2rmvdl/we_want_you_to_apply_for_flair_the_panel_of/
p.s. we're not awarding flairs for April Fools content