It may sound boring for many, unnecessary even, or like something that takes the fun out of it, but if you want to write something good and to avoid the infamous Writer's Block (TM), I at the very least recommend you to plan a very rough plot line out before you write. Plus, the planning can be fun. Not to mention, you write Fanfiction, you don't have to do world building, so the planning isn't that much.

Now, I want to mention that there are three kinds of writers.

The discovery writer, they plan nothing to not really, they write as they go and tend to have Writer's Block (TM) more easily. They are the ones that tend to discontinue stuff more likely. They are mainly Fanfiction writers.

Than there is the Architect Writers, they plan a lot to everything. They tend be not as easily caught in The Writer's Block (TM). Most professional authors are Architect Writer's, but they kind of have to.

Last but not least is the Hybrid of these two kinds, someone who does plan stuff, but not that detailed. They still add and change stuff as they write. That's what I would call me now.

Now, let me tell you, as a Fanfiction writer, unless it is a very heavy AU, you don't have to plan that much. The world building is mostly done. At most, you need to research a bit. So, you mostly need to write an outline of the plot. My "outlines" are mostly bullet points. Though, a while ago, I started to write more descriptive texts that go more into Detail.

That's mostly because I started to write more original works, so my general style and preparation habits change.

Now, the biggest reason for this pre-work is, when you know what comes next, you don't have The Writer's Block (TM) as easily. You already know what comes, you just need to write it down as a nice text.

Plus, planning out the plot is a good way to brainstorm ideas. The brain often needs a push into productivity. Which means you need to force yourself to just write sometimes.

Not to mention, if you take a break, a hiatus, and return to write, without any notes you probably would forget the plot and end up abandoning the Fanfiction. With notes, you probably are good to continue after either re-reading the notes and what you've written so far and maybe re-write what you had so far to get the feel for the project back.

Now, a plot, the events that make up the story, they are connected. Many writers just have events happening, out of nowhere. Just random things happening, back to back, no connections. These Fanfictions are rather boring, as they don't have a real plot.

The plot starts with an event, this event has a cause and effects. The characters react to it. This is what a plot is, things happen for reasons, these things have effects and characters react to them.

It's like history, everything is connected, everything has a cause and effect. If not, the Fanfiction you write is boring, there is nothing the reader can be invested in.

Just remember that, when you plan the plot. Think how character X may react to it? You need to consider the characters traits. An aggressive and a passive character will react different to a cheating lover, for example.

What event starts your story?
If you write a retelling of something, i.e "What if Zuko is the Avatar?", maybe you don't need to change that much. Though, you need to find explanations hat don't come out of nowhere. It has to fit the Lore.
Though, with my example, you need an explanation how the next Avatar comes from the fire nation and, as Zuko needs to learn how to air bend, find a way to have a living air Bender.

You need to explain, like said, things happen for reasons, these reasons have causes. These causes have many effects.

If it Isn't a rewrite with a change that causes changes in the rewrite, you need to come up with an event.

This event has to be logical, be something that could happen in the universe you're writing for and it will have effects. What are these, are they causing effects of their own?

All these thoughts, the chains of events, of effects, write them down. Sort them. With that, write an outline.

Depending on what kind of Fanfiction you write, you may need to do research, or fill in or change Lore. Many AU's are very different from the original IP, these need a lot of planning.

Now, when you have the plot lined out, the level of detail depends of course, you may start the actual writing process.

It may sound annoying, but it isn't. It is actually fun.

Plus, an already planned plot will give The Writer's Block (TM) a hard time. Isn't that worth the (in most cases low) effort?

Most stories are build in a scheme. The exciting event, exposition and build up, until the big climax, the solution of the plot, the end of a story. The solution needs to be logical, it has to have a reason that fits into the Lore.