How to Write Good Fanfics

1: The Characters and Story

One of the key things to any piece of writing is the characters. The characters are what the reader is supposed to observe and grow sort of an emotional bond with.

When dealing with original characters, it's important to give them all distinct personalities so that every character seems like a different person.
For example, one person might be peppy, cutesy, and always energetic. However, you can't just fill the story with people with those personality traits, or else it'll be kinda boring, kinda like if everyone had the same job, face, and routine.
Additionally, you don't want to abuse the character's main personality traits, or else they start to get annoying or unlikeable.
Be sure that the characters develop as the story goes along, as a character without any character development can seem a bit dull.

When you're writing a fanfiction, meaning you're writing your own story about a certain franchise or fandom you love, it's crucial to keep the characters the way they are in their respective franchises, unless you're making a drastic change to the setting of the story, or the change in character would make sense in theory. (such as Chief Bogo in my Bohauser fanfiction, where he acts way different than he was in the movie because he's in love.) Otherwise, make sure to keep the characters the same. That way, you won't alienate the fans of the franchise/fandom and you honor the characters' creators.
Take Clawhauser for example. Clawhauser can be described as a happy-go-lucky, chubby, loving cheetah who has a huge heart (and gut) and works in the police force.
So, if you gave him like, huge abs and muscles and a dark and edgy personality, people will probably not want to read what you're writing. (I certainly wouldn't)

As for the story, you should always avoid making it cliché- pretty much akin to many other fanfics- or worse, blatantly ripping off another author's story and claiming as your own.
Usually, a story follows a formula, but for the sake of time, I'm going to explain that in a later chapter.

AVOID IMPLEMENTING "MARY SUE" CHARACTERS INTO A FANFIC
Mary Sue characters are generally OCs or self implements that either changes or warps the story around them. This, in a way, bastardizes the story that you're writing a fanfiction on, just like changing the characters drastically bastardizes the character and their creator.

If your story has smut in it, try not to make it a key concept in the story.
Yes, couples have sex. It does happen. But don't make it happen every other chapter. I learned that mistake myself.
Plus, adding a smut chapter to a story focused more on the love between two people can seem really distracting and out of place, almost making your audience uncomfortable. It's safer just to leave a smut chapter out in those situations.

One last thing.
Make sure that when you're writing a fanfic, the setting is familiar to them. You don't want your audience thinking, "Well, this is a (insert franchise/fandom here) story, but I don't really feel like I'm in the same world as it takes place."
Your audience will like what you're writing better if they feel right at home in the setting that you've placed. If someone were to write a Zootopia fanfiction, but I don't feel like I'm in Zootopia while reading it, I may not like it as much as I would if I did feel like I was there.

Well, that's all I have on that subject. More coming soon!