If the idea you note down is any good today, it'll be a good idea tomorrow too. And a year from now.
If you have enough good starting points, actually spending some time writing out an essay from them becomes easier. Remember you'll probably want to do at least three re-writes if you're hoping the result is going to be any good. Lots of people don't do that -- and it shows. Most half-decent blog posts would've been a lot better if the authors took the time to rework them a bit more. Or, according to Hemmingway: "“The first draft of anything is shit.”
So with the caveat that I don't actually write much (yet?), the best book I've read on writing is: William Zinsser's "On Writing Well":
Highly recommended for anyone that have to communicate in writing (ie: everyone).
> I don't have anything to say to people who know less than me, because explaining obvious things seems boring, and I don't know what to say to people on HN/LessWrong, because I feel like they are smarter than me and already know everything I am about to say.
For essays, it can be good to write for yourself. To yourself, or someone much like yourself, but someone who's perhaps not yet encountered one particular idea, one particular technique -- one particular subject.
That usually gives a good framework for avoid "talking down". Write to yourself of one, two or five years ago. There will be many that don't have that last year, years of experience and circumstance that led you down the path to were you are now. Perhaps such a perspective makes it easier for you to share something?
I mostly use Colornote to keep track of ideas, such as app/application/project ideas along with a couple of bulletpoints (eg: Reinvent email: look into alternative client/server sync such as jmap; store email in normalized sql db?; document db?; store attachments based on content hash? (free de-dup); Store email body as same? (Good for multi-user server support for mailinglists ... etc))