Ask HN: How do you write specs/requirements/design documentation?

I work at company where we don't do any of this. Which leads to more iteration and wasting time.

I'm trying to introduce writing specifications as a start, but I'm worried my knowledge is quite dated. I learned it in school but never really applied it in the real world.

Each project is of course different: The projects I work on are web applications and rest apis that depend on multiple services. Current web app I'm working involves 10 different systems.

I was wondering how you approach writing these kinds of documents? Or how you in general keep an overview of your project?