I know that this is super duper complicated, and there are entire books about this and I'm trying to stuff it all into one chapter, but I'm slowly running out of ideas, so you get what you get, and you don't get upset! Or that's what my second grade teacher would've said.
You need to get the characters right.
In fact, you should take some time to sit down and list every single little detail about every character. Because you need the characters in the fanfic to be accurate, down to the last detail. If you don't, instead of a fanfic, it will be just a couple of character running around with the same names as the originals. So yeah. You really need to work hard on the characters.
Next.
Change the plot
You can't just have the plots be the exact same. You need to switch up some stuff, because if you don't, the plot wouldn't be yours, it would be the author's. And if you put in the super surprising part that was in your book in your fanfic, then everyone will be expecting it, and they wouldn't be surprised.
Okay, that was probably the shortest chapter I've ever written. But I'm in a hurry, and I didn't want to leave you hanging. So yeah. A couple little tips. And I probably will touch on fanfictions later, but I'm in a bit of a crisis right now:
I'm running out of ideas. So if you could tell me something you need help with, please tell me in the comments. I'll try my best. But I probably won't update again until I get some more inspiration.