The first step is getting the cannon up to date.
Read or watch the original material fully, do not skip steps. You are writing this to fans who like said original material so much they are willing to spend time searching and reading fanfics about it. This is a pit of passionate fans, do not underestimate their response to something that goes against cannon without prior warning. Dialogue is crucial everywhere, especially here.
On another note, assume your readers know the original material. Don't go too much into describing a character and their lore right off the bat. Only do that after a particular event involving them personally on a deep level, should it ever come. The exception to this is an original character or something not in canon. Most people who read fanfiction are up to date on the series, so there's no need to repeat what people already know.
Any violation from cannon you do, establish that right from the beginning. Don't betray your readers and the time they invested in it. Give the proper warnings and tags. It also helps the readers to find your fanfic and even filter the kind of readers you want right away.
Also, read the tropes, genres, etc, you are trying to pull off. Not only it serves as inspiration, but it can also help you try to not fall into cliches and be more original.
The second step is deciding your cannon relationship. (Mostly from tvtropes).
[Adaptation]: Taking an already existing story and adapting it to text. Generally works best with plot-thin Video Games (where some extrapolation to fill holes is fine), but it can be done with other visual formats to decent success.
If you want to adapt stories that are already text-based you should make sure to examine the events and characters of the original from a new angle. This is very often done by changing the perspective. What did the villain feel in the end, during her crushing defeat? What did the Innocent Bystander witnessing the event, make of it?
[Continuation]: A story that takes the existing canon and goes further in its same style. This could include filling in details that weren't shown in the original work (e.g. an off-screen character's struggles that were only alluded to on-screen). This is one's best bet for a wide audience, but it requires a decent understanding of the entire work.
[Divergence]: Taking an existing story and asking "what if?" on a crucial detail. What if the antagonist was right all along? What if the protagonist's parents didn't die? What if one or more of the characters was gender-flipped? This format can offer a much better understanding of the canon as a whole by showing how a single change would alter the dynamic.
[Elsewhere]: Using the setting minus the main cast of characters as a backdrop for some new characters and their adventures. Video games, Tabletop Games, and especially MMORPGs lend themselves very well to this. Many games like Dungeons & Dragons, Warhammer 40,000, and City of Heroes are almost exclusively designed just for this purpose. One still has to follow the established rules of the universe, though.
[Alternative Universe]: The inverse of Elsewhere, this involves taking existing characters and casting them in entirely different roles and often a whole different setting. Different from Divergence in that it completely discards the main plot in the process. This is a fairly controversial format due to the tendency of the characters to be In Name-Only. If writing an AU fic, remember to stay true to the characters' personalities and gimmicks despite their new setting.
[Crossover]: The fictional universe fuses with another one, whether naturally (as in it was always that way and not a result of an event), through portals, a sudden contact, or anything that explains said fusion. Cannon becomes nothing more than a guideline in those, and it should be treated as such. A crossover, depending on the degree, can have more extreme changes than an alternative universe.