Ask HN: How to make clear and legible diagrams

Diagramming software and tools (e.g. PlantUML or even Inscape) are a popular subject on here.

I've played around with them and read their tutorials. But what's missing for me is information about what makes a diagram clear and readable. A lot of the examples are inscrutable (or at the very least ambiguous) if you don't know the conventions of the specific diagram used (the meaning of the icons, the arrows, the text inside and outside of boxes, ...).

Are there any resources that cover the basics of diagramming (starting with the simplest flowchart), regardless of the software/medium used? Once you know how to make a clear diagram with pen-and-paper the most interesting work is done and all that's left is (optionally) converting it into digital form.