Sources. Got to know sources for information from the original. Facts. Any fanfiction can't be very well if the facts are all wrong.
Perception. If the main characters young, then write what they say, feel, think, and see alittle simpler. But still keeping the surroundings and actions very very detailed.
Cover picture. Worry about that last, or if your very skilled at drawing on paper or on the computer, then by all means do it.
Character build. With any story, you need the characters to be rich and juicy, filled with realistic traits and backstory.
That's how a good fanfiction, or good story in general is crafted. ;)
