it depends on the project - smaller ones can be done in one day, but things with 10k+ words (or like. the doctor fic, or something i've written/havent posted that is longer like that) i do have a whole process.
i will start with an idea or scene i really want to get to, and then create a rough outline on how i would like it to go. there are usually some snags when i actually write things out - for example, i was stuck on how to injure mika in the doctor fic. i knew i wanted him to get stitches (literally what i had written down for the outline) but didn't know what the cause should've been. i had a few different scenarios written out, and i talked it out over dinner with a friend!
i either have a spreadsheet (i have a REALLY intricate one for the fantasy au) or a bunch of bullet points written out (like the vampire fic im working on). i don't usually pants something unless it's a shorter oneshot - i like to know exactly where i'm going and really love to sneak in some crumbs for people who catch them!

once i had a chapter done and the majority of the next one finished up (or when it was completed for the shorter stories), i sent it over to my beta reader and he would mark it to hell and shame me for my excessive m dashes.
i still owe them the world