How it works
Setup takes a few minutes. The rest is reference — come back to it when a HUD does something you don't recognise.
Where to start
If you haven't installed anything yet, read getting started and nothing else. It's five steps and it's the only page you need before your first game.
Reference
The HUD reference is the page you'll come back to. Each overlay appears when the round needs it rather than all at once, so a quiet HUD early on is normal — that page says what each one is waiting for.
Automation is worth reading before you switch anything on. Everything that clicks for you is off by default, and that page explains why, plus exactly where Riot's rules do and don't cover it.
Elsewhere
The guide is the illustrated walkthrough — every screen with a screenshot, better if you'd rather see it than read it. The changelog says what each release changed. Anything the docs don't answer, ask on Discord.