This package will also automatically install gltFast Plugin if it's not already in your project. If there's an issue you can install it manually with package manager: com.unity.cloud.gltfast.
Subscribe to Plus plan. There are several places to do it in the app, here's one of them:
Go to https://app.sloyd.ai/user, scroll down to the API section, and press generate API key. Then you will see a client ID and a client secret field like this:
Back in the Unity project, open the Sloyd Tool widget and enter the Client ID and Client Secret. A green light will indicate that authentication succeeded and that you can start using the tool.