Step 1: Edit Wikipedia to insert a fake "fact". Choose your fact carefully: it needs to be unlikely enough that your mark will bet against it, but not so crazy that it will provoke obvious incredulity ("Prince Philip, the Duke of Edinburgh, is a Furry"), causing the mark to doubt the veracity of Wikipedia. You also need to be sure that your edit won't get quickly reverted, so stay away from entries that are closely watched, controversial, or recently in the news ("in 2014, more goats were killed in rail accidents in the US than people").
Step 2: Choose your mark and place your bet.
Step 3: "Prove" your claim by looking it up on Wikipedia. Collect your winnings and leave before the mark checks other sources or your Wikipedia edit gets reverted.