Google OAuth Setup for Webapp

My visual-development driven web app is built with plain HTML, CSS and vanilla JavaScript, and it now needs its own “Use a custom OAuth from Google console” flow. I already hold an active Google Cloud account, yet I only have a partial grasp of the OAuth screens and credential types, so I’d like an expert to take me from configuration to a working, tested sign-in.

Scope of work
• In my Google Cloud console: create or tune the project, configure the OAuth consent screen, and generate the correct Client ID/secret for a web application.
• Add any required authorised domains, redirect URIs and scopes so the consent page is clean and brand-matched.
• Drop the resulting credentials into my existing HTML/JS code (or advise the exact snippet) so users land on Google’s sign-in, grant access, and are returned authenticated.
• Validate the entire flow live, confirming tokens are issued and basic profile data can be read.

Deliverables
1. Google Cloud project fully configured with working OAuth credential.
2. Updated or clearly documented JS snippet integrated in my codebase.
3. Short hand-off guide (or recorded screen share) so I can maintain or replicate the setup later.

Acceptance will be based on a successful login from at least two separate Google accounts and a clear, reproducible setup.