THNK is currently in early stages. It is still usable and a much better experience than building multiplayer by yourself, but it may have bugs or features that are important for you that are not yet available. Be aware of this when considering THNK!
THNK is a GDevelop framework, so you will need to have GDevelop downloaded. It is recommended to have some base GDevelop knowledge before getting started with THNK. You can find resources to learn GDevelop on the GDevelop wiki.
To get started with THNK, the first step is to download the core THNK extension. As of the time of writing this, it is not available yet in the extension gallery, and has to be installed manually.
To import it, go into the Project Manager, click on Functions/Behaviors, select "Create or search for new extensions", and finally click "Import extension" button at the bottom left. It will open a file selector that will allow you to open the THNK extension file.