When pairing up your protagonist with a really smart person (computer genius, brilliant mathematician etc), make sure you have at least some background research done on the issue.
Make characters with different minds. Not everyone is pro LGBTAQ+ so don't try to make it out that way just because you support the rainbow community.
Involving current events is a great way to ensure you have smart people reading your fanfiction.
People who don't use grammar, spelling, punctuation etc are not as attractive as you make them out to be.
When foreshadowing, do it well If it doesn't fit in there, foreshadow when the incident/event being foreshadowed is actually required.
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Tips For Writing Fanfiction
RandomExactly what the title says. (If you have questions, you can PM me. I'll try to help. Or, you know, take any insults you want to throw at me.)