Hi, bbies! Just posting this chapter since I frequently get asked this question! And I will do my best to explain. 

So basically I have no clue... For the most part, when I read over my plans or even do so it's just basically to get the book going. I don't think too much about what happens in filler chapters I usually try to make those either fluff chapters to give you guys a break from any angst I post or it carries on a bit with the angst but doesn't get too deep just to again give you guys a little break. 

I'm still working on trying to make other characters likable (even with some of the villains for at least I try). But normally plots and certain chapters (like smut chapters) are talked through with my wife or I base them off on daydreams or actually dreams. And sometimes while writing I'll pause and just try and imagine the scenes like I'm watching a movie. 

From there I write down the things I imagine or are planned and go through and do my best to catch errors or revise to make them sound better. (I know I still miss errors) And even if I'm not writing, I will get brief ideas that may sound good for a book and will write it down. I'll give you an example of a book I hope to work on at some point: 

So this book is an Aizawa x Reader:

To summarize, Aizawa and Reader have been friends forever, he was a little more reluctant to be friends with her and even comes off a bit rude, but because of Present Mic and Shirakumo, they were all friends. Reader moves away to America after losing her quirk ;). Anyway, years pass and she gets an opportunity to work at UA. 

Now for the scene: 

So it's after a night Aizawa and Reader go out drinking with Midnight and Mic, the two are kinda tipsy but not like wasted. Anyway they back to Aizawa's room (Reader wanted to make sure he was okay). Aizawa invites her in after he tells her he's got all these assignments to grade, so she offers to help. Well after he shuts the door and heads for his desk he turns around and watches her take off her coat revealing her in a silky floor-length dress, most of her back is exposed. And he walks up behind her and from there things get sexual and they do the devil's tango. In the morning Reader will leave before he wakes up which I had planned to have more fun with this. 

So yeah that's basically all I got as to how I write so well? But another thing I think helps is just keep practicing, proofread, and ask friends to check over your work (if you trust them enough). Mistakes happen and if you catch them while reading over your book just go back and fix them. I think I also just read a lot and tried to use similar writing styles from popular authors that helped me? But yeah! If you have any other questions let me know!

P.S if you want something that will catch your error you can also use Grammarly!