Good advice for starting a light novel?

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Good ones!

But i'll add something to the line myself,by adding something i've already done:

I've praticed my writing style, by rewriting scenes from movies. I just normally wached the movie, and then started to rewatch it,descride the scenes by writing and adding the existing dialogs.
If the movie already has a novel or a book, you can compare your outcome with it.

That's pretty good practice if you ask me. Even better is taking existing characters and maybe even existing scenes and writing completely new lines and take the story in a different direction. This will make you more proficient at actually writing characters in a consistent way and make you more attentive to details and themes, whereas the imagination part is something that will come after you study a wide range of different characters. But I definitely like what you're doing now, especially if you pay attention to the themes and try to express everything that the movie cannot do with ease (or at all).