Thorough answer:

1. Create a Google Cloud Project: https://console.cloud.google.com/welcome

2. Set up the OAuth Screen: https://console.cloud.google.com/auth/overview

3. Add yourself as a Test User: https://console.cloud.google.com/auth/audience (scroll down)

4. Copy your Project ID: See the Homepage of your Project or see this link.

5. Go back to your Google Sheet Script

6. Go to Settings: https://script.google.com/home/projects/\[SOME_LONG_STRING\]/settings

7. Add your Project ID under "Google Cloud Platform (GCP) project"

8. Try running a function in Google Sheet Script -> this will trigger the OAuth screen

9. See your logs here: https://console.cloud.google.com/logs/query