Sometimes I just start writing and see where it takes me. Then I find a way to fit it in to the rest of the story. But sometimes I'm very bad at this, and instead of getting into the zone where the words and concepts flow naturally, I get distracted and start overthinking things.
Things like: am I using too many words? Not enough to get across the concept? Did I capture this characters nature properly, or is it just tropes and simple one liners I cobbled together in the hopes that it makes something coherent?
Am I remembering to not just describe the next thing that happens, bit to add personalization to the Narrator or POV character?
Sometimes I just get overwhelmed with insecurities in my writing that it takes me out of the flow entirely, preferring to distract myself and procrastinate rather than being confident in my own writing skills. Sometimes I only get a few sentences down in a day, and the next, nearly 4k words.