I kinda don't, but the times I do succeed usually revolve around consuming media related to what I want to write. Fantasy work comes easier when I'm playing games and watching movies that contribute to the style I want to build; you probably shouldn't be watching The Mandalorian and then trying to write a LOTR fanfiction in the same stretch. Runners up include goalsetting/forcing myself to write even if it looks terrible (at least you have something to proofread) and ridding yourself of things that would distract you from the necessary mindset.
I similarly am poor with word vomit, but I'll prescribe a bit anyways; the idea that works best for me is to have an outline of what you want to touch base on in what you post. Don't prewrite, but just note the sorts of things you want to hit; cover what came before, call it good, run through actions in the order that would be relevant to your PoV, basically just arrange the notable details and hit them one by one. You shouldn't be repeating yourself if you have a grasp of time flowing in the scale of your content.