I went through something similar just trying to get my NEST to work in home assistant, thanks to HA these are the well documented steps, and each step has multiple sub-steps...

  - Create and configure Cloud Project [Cloud Console]
  - Configure OAuth Consent screen [Cloud Console]
  - Configure OAuth Application Credentials [Cloud Console]
  - Create a Device Access Project [Device Access Console]
  - Enable events and Pub/Sub topic [Device Access & Cloud Console]
  - Link Google Account

It ended up that the linking would not stay persistent, a waste of $5 and many hours.