No need to install anything first, just start with the Beginner's Guide, and work your way through three guided tutorials on the syntax of writing Yarn scripts, installing a text editor to write Yarn scripts, and then integrating Yarn scripts with your game, and installing Yarn Spinner as a package into your game engine.
We'd also recommend joining the Yarn Spinner Discord, where you can show off your work, and ask for help.
Yarn Spinner is not just a single project or tool, but is a collection of projects and tools. It consists of a language to write your narratives in—Yarn—and tools to display, and use your Yarn in game engines, like Unity, and tools to translate, localize, and connect voice over to your stories.
Yarn Spinner is provided as free, open source software by the team at Yarn Spinner Pty. Ltd. We want this to continue being our full time jobs. You can help by hiring us to help integrate Yarn Spinner in your games, build specific features you want, or consult, and by backing our Patreon.