How should I write things?

Encrypted, backed up, and moved from computer to computer as circumstances evolve.

Not the main question here, of course, but whatever information you seriously intend to still have around in 10 years, you need to allocate some attention to safeguarding its journey through time.

The comparison that others made to program documentation is another aspect of this. You need to include enough context to ensure that the words still communicate in 10 years time what they mean now. How much of yourself 10 years ago, or whatever timescale you're thinking of, can you remember? What would you wish your earlier selves' journals to contain?

I try to write my journal for me, about ten years from now. So, I don't spend much time explaining who people are that I know very well, or what my overall situation is -- but I do spend quite some time trying to express mental states, because I know that how I think now differs vastly from my thinking ten years ago, and I expect similar changes into the future.

On the other hand, I've had lots of experience with trying and failing to understand what I've written in programming and mathematics, so I've internalized the fact that future-me might not even understand an explanation of things I think are obvious right now. ymmv.

I write my journal as though I'm talking to myself in the future. If it's a detail I think I'll forget, I write it down. I have a general rule that I don't show people that journal, simply because I know I'll be less honest if I do.

Technical notes: I make a new document every six months, and I write in it pretty much every other day. Usually I write about my state of mind, examine why I'm feeling whatever it is I'm feeling, and what I did that day. It was originally an attempt to remember things, since I have a notoriously bad memory for dates, events, and the like. The current six-month diary is kept on my computer, password protected. I made it accessible because I don't want minor inconveniences to prevent me from writing in it. The old ones are kept on a flash drive (encrypted, password protected) and I've put it somewhere safe. Six months because three is safer in terms of "if someone steals my computer or something, keeping three months is less information" but its a hassle to get the old versions out, and I use it to monitor how I change over time. A year would be ideal for that, but that's too much information to keep in one place, so I settled for six months.

To the point: Write frequently, and to your future self. Don't make it so accessible that anyone could get to it, but have it accessible enough that you will write in it.

Well, you are writing for yourself, but you aren't writing yourself right now who has the event readily accessible in memory. Do you think that you will remember why you decided not leave apartment after seeing the blue civic in a week? A month? 5 years? If no, then fill in the details. For censoring, just make sure the file or physical thing is safe, or you can pretend that you are communicating to your future self through some insecure means and need to encrypt it.

I would say that a good heuristic (in my experience) is that you should write it as you would write fiction. In other words, pretend that you are writing it as a story for publication. (Or maybe publication in a very similar alternate universe where the events you were writing about did not actually happen.)

Think about how much information the reader is typicality given in the standard novel that you read. It is a balance between the factors you mentioned. Enough information for readers to know what is going on, without the burden of irrelevant details.

It depends for me on what my purpose in writing is, when using this technique.

If I'm trying to make sense of a complex process, I draft as though I were writing for someone who had to implement that process. If I'm trying to make sense of my emotions, I draft as though I were writing for my therapist. If I'm trying to make sense of moral judgments, I draft as though I were the various agents experiencing the consequences of particular judgments.

Typically, when I'm done with that process I go back and revise for a general audience and post to my personal blog, but not always.