For legends and prophesies you take one of the basic story lines and run with it. It will be basic and mostly lack detail and description that you would have with something that is happening or something that happened and hasn't had time for story tellers to have their way with it yet.

With legends you take it over the top. How over the top depends on whether you want the legendary character to show up or the legend to come true again.
With prophesies you make it vague. The best prophesies can "come true" many times.