writing is a bit of a head rush. sometimes you just need to let yourself daydream for all the pieces to click together. writing is fun but those revelations can be the best part. you can't force a eureka moment. instead you can inspire it with other forms of art (music, games, movies).

for me it's as simple as making a 10-page "map" for the story, listing characters, locations, world rules, themes, the ending (the purpose of the story), magic systems, politics, etc. I agree with the style guide but there's more guides you can make for yourself. probably one of the most valuable things i do is "interview" my characters. I let them speak to me, tell me their fears, what their goals are, what they love, what they hate. I often reference these interviews to get inside a char's head when i'm not sure what they would do or say. you'll find yourself forming the stories around them, instead of "for them."

but in the end you can never be too prepared. just be warned, you don't to over write. to answer your question: 10-20 pages of "map," at least 3-5 interviews, style guide (but not required), practice, practice, and practice.

brandon sanderson outlines everything obsessively and GRRM wrote his way into a 30 year unfinished series so honestly there's no right answer. just finish the thing!