Hello,
It's kind of easy to start, as they said on the top you need to create REST API tenant code (Key) in the advanced settings, then you'll need a admin user and an admin role. I'll suggest to create one specific role for the API access and the operations you want automate.
In order to connect you need to put tenant code and user-pass on the header of the request. user-pass needs to be encoded.
I'll recommend to use Post-Man or Insomnia apps in order to test your API connection.
Regards.