Plans! Are! Important!
I honestly cannot stress this enough! Whatever you're writing you should always have a plan for where it's going. It doesn't have to be an incredibly detailed and thought out plan you don't even have to always stick to it. Don't wing it, if it's going to be a novel of full length fan fiction. Please don't wing it. It's easy to tell when people have (usually) and it just looks a little sloppy.
I always feel like it's a good idea to plan by chapter, what is going to happen in each chapter, on a rough scale. You don't even need to be that detailed, but what you do need is a beginning, end, problem and a resolution for each problem there is, more on that later.
Fiction generally follows a plot for no matter what sort of novel it is, or at least, well written fiction follows this sort of structure. Now you may think, how can all good fiction follow the same structure but be so different. This is an incredibly vague structure, INCREDIBLY VAGUE.
And it goes a little something along these lines.
1. Character
The easiest way to start is always by introducing characters. This may be introducing an oc but generally in fan fiction it should focus on the relationship between characters. In the case of an oc or reader insert it's helpful to include a little bit of background information spread throughout the beginning of the fiction. Of course, that's not to say introduce all your characters at once. It's more along the lines of, establish the protagonist and perhaps another character along with that. Or establish the protagonist's relationship with a love interest.1.5 Setting
This is interchangeable with the character, or can even be established alongside the introduction of a protagonist. This is the setting of the world the characters live in, is it a school, a university, a medieval kingdom?2. The problem
A problem is established that the main character/s need to overcome. It can be anything from a physical rival or evil to a simple bump in a relationship or even getting characters to form a relationship. The problem should be established pretty early on, it can either be a problem that has been plaguing the main character/s for some time before the start of the story and only now have they started to act (this is hard to pull off well and I do not recommend using this method unless you know what you are doing), or it can be a newly established problem. Try not to overlap the problem with a character introduction. It can be done, but don't cram it in all at once.3. Solving the problem
In other words, a resolution. Now this can go one of two ways, the problem can be solved on the first attempt, or an attempt to resolve the problem can result in the problem worsening or just a straight up failure to fix whatever is going on. The former is good if you're thinking of writing a shorter story.The structure of a plot isn't limited to these four parts, there can be multiple problems, problems within problems or more than one attempt at a resolution. Or perhaps resolving one problem while another is still going one. The great thing about this structure is that it's so flexible. But ALWAYS have a resolution for a problem. Not fixing what went wrong can leave readers feeling unsatisfied. I have only seen this done well a few times, such as in the end of 'A Series of Unfortunate Events' even then, 'The Beatrice Letters' was written to leave questions open but tidy the series ending up slightly.
Of course it's great to have cliffhangers, just don't allow your ending to be one. Always tie up your loose ends.
I'm only going to be brief on oneshots, as they are exactly what they're named. It's not really a story, it's a snippet of a story, with the problem perhaps hinted at, but not fully addressed. This obviously does not follow the structure, although it can to some extent. For a ficlet a minor problem can be put forward and resolved but using a major problem can make the one shot too full and seem rushed at times.
The beauty of one shots is that they're incredibly flexible and plot wise, you're free to do what you want, have a brief idea of what you want in mind. Perhaps a beginning and an ending to (how you want it to start and at which point you should end it, instead of going on and on). Just have fun writing it!

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Writing Tips and Tricks
RandomFrom aspiring authors to first time fanfic writers. From those who are new to writing and want to learn about the basics, and to those who have been telling stories for years and want to brush up on techniques. This is my personal take on writing...