I’ve been thinking of contributing to foss projects with more than testing and bug reporting. I want to contribute to mainly open source game engines. There are a few that are mature and underrated, but lack any documentation for a user to get started. This is why I decided to do documentation, but I don’t know where or how to start.
Is there any general rule/guidelines for getting started with this ? I’m looking for anything that can help a beginner documentation writer


So they have existing documentation? If so see if they have a style guide for how they want info presented. Assuming you are writing to official docs.
You can also look at other projects how their info is presented