Create a new project on Google Cloud

Go to APIs & Services then Credentials

Click [Configure Consent Screen]

Fill info. Use the ChatGPT prompts in pages /tos & /privacy-policy to generate yours automatically. Add userinfo.email & userinfo.profile to scope. Add yourself as a test user. Submit.

Go to Credentials and click [+ Create Credentials] then [Oauth Client ID]

Choose [Web Application]. Add http://localhost:3000 and https://your-project.supabase.co to Authorized JavaScript origins. Add https://your-project.supabase.co/auth/v1/callback to Authorized redirect URIs (if you're using a subdomain like www, make sure to add it too). Click [Create]

Copy paste the Client ID in GOOGLE_ID and Client Secret in GOOGLE_SECRET to your Supabase dashboard (Authentication, Providers, Google)

In Supabase, go to [Authentication] then [URL Configuration] and add http://localhost:3000 to [Site URL]. Then add http://localhost:3000/* to [Redirect URLs].

Go to [Oauth Consent Screen] and click [Publish App] then submit for verification. Click [Prepare for verification] and fill the missing information. Google will email you and you will have to reply to start the process. You'll need to have your domain verified with Google Search Console. You can go ahead and do that now. You can already login with Google on localhost. On production, it will work too but show a warning until you're verified (takes a few days).