1) Observe the world, pay attention to things.
2) Jot down snippets of observations/ideas in a single Google Docs document (available on almost any platform). Don't try to organize it. Keep it really simple. The key is to keep it low effort and frictionless.
3) Look over the Google Docs document from time to time. When you have enough ideas coalescing in a cluster/theme, feel them out. When the cluster feels like it's ready i.e. you feel an emotional connection to it, start writing. Important tip: don't feel you have to finish writing in one sitting -- you can stop, let your writing sit, and keep re-clustering until you have something that is worth putting out there.
Gerald Weinberg calls this the Fieldstone method  -- the basic idea is to continuously collect, and only write about what you care about. I think it's a useful approach. I personally don't find the streams-of-consciousness approach very compelling (didn't like Virginia Woolf's Mrs Dalloway). Streams-of-consciousness is good for very artsy-fartsy, avant-garde type works, but to me good writing comes from a lifetime of collecting experiences, observations and ideas and coalescing them.
On mistakes: I used to have a lot of anxiety about my grammar/mechanics because I'm sloppy, but I learned from an editing coach that "everybody could use a good editor". Focus on getting your ideas right -- if it's worth putting out there, you can always hire an editor to help tighten your copy. Development editors will help you build your piece, line/copy editors can fix your style and mechanics, fact-checkers can help with research on veracity. There are at least 9 types of editors, each for different stages of writing . But you have to first have an idea that is worth communicating.