Technically, I'd say you should learn everything in this book before you start writing, but these will help specifically with pre-writing.
The absolute number-one mistake you can make is neglecting to plan your story. You think that outline you had to do for English class last week was bullshit? Well, actually, that kind of stuff can seriously help you not fuck up. If you write a story, every single thing you do comes out of your own brain. You are the one typing the words on the screen. If you want to write a good fanfic, you have to make sure that it's as perfect as you can make it. That's why pre-writing is just as important to the process as the writing itself.
The first step is to get an idea of what you want to do and where you want the story to go. This is important: do not steal anyone's ideas. Not only is it fucking rude, but it seriously detracts from the appeal of the story. MaiaMDMA wrote A Splitting Of The Mind, and suddenly, there are five thousand fics about schizophrenic Gerard Way. You want your story to stand out. The best way to do this is to come up with an original setting, or at least one that isn't overdone. If you really want to set it in a "classic" (or cliché) place, i.e. high school, make sure there's some sort of twist.
Once you're confident with your idea, you either start on research or planning. You only need to do research if your fic will deal with an environment, activities, or anything else you aren't familiar with. If you claim that one character has panic attacks but actually exhibits behavior of moderate anxiety, there will be trouble. I once read a fic where the author was obviously in elementary school because they referenced recess and walking in straight lines in a high school. Having a bunch of errors in your setting, characters' attributes, etc. will only harm the appeal of your fic along with your credibility. If you're not sure of something, look it up. Bullshit essays as much as you want, but don't bullshit your fanfiction. No one will read it.
Planning can be done in a variety of ways. Sometimes I make a timeline or a calendar, sometimes I write out what will happen in every chapter; sometimes I do both. Either way, you should know exactly what's going to happen in your fic. If you make it up as you go along, I guarantee that you'll get impatient and have Mike fuck Tony four chapters too soon. Planning is the logical part of the process. You can assess whether it progresses at the right speed, whether the events are in order, eliminate plot holes, and get a general idea of the meaning of the fic. It can be beneficial to stick in a few filler chapters, which add character depth.
(NOTE: Common misconception, a filler chapter isn't a chapter without romance and shit. It is a chapter that is detached or non-influential of the plot).
One benefit of pre-writing is that you can input foreshadowing. I really fucking like foreshadowing. I don't mean the "little did I know" shit, I mean the full-on Fight Club "I know this because Tyler knows this" gold that you don't even know is foreshadowing until the book is over. A fic shouldn't just tell a story, it should fuck you up a little bit with all the tricks the author inputs.
Another tip: It's okay to torture your readers to an extent. Secondhand embarrassment, shit luck, whatever. If you want to write only shit that will make the reader's heart flutter, do a oneshot. Too much of a good thing exists and it is a problem.
The last thing about plot: If you input a plot twist, it should be unexpected. Not stupid and abrupt; it should make sense, but don't give too many clues. If the reader knows it's coming, especially more than a chapter away, it is not a plot twist and it isn't exciting. It's so annoying to read, "Yeah, for some reason Ryan gave me his favorite guitar yesterday," and you know Ryan is going to attempt suicide. Basically, just be careful.
The next step in planning your fic is to develop your characters. I usually plan every somewhat significant character's identity pretty thoroughly. You want your characters to seem like real people, so every single one of them should have flaws. Now, I don't mean the usual depression or self-esteem issues. It's okay to write them, but I think that, at this point, they're very overdone and you should work toward something more original. I recommend writing a short bio for each character (for your eyes only; don't post them) containing information about their personality and facts i.e. age, pet peeves, whatever you want. In your actual writing, you don't have to include every detail you put down in your bios, especially for more minor characters. Generally, your writing will end up conforming to your plan without too much effort. The bios are just there to help with keeping a solid, consistent idea.
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How NOT to Write a Fanfic
FanfictionSorry if I piss you off. A lot of this is opinion.