Documenting how to setup CORS with an Example for ease of use.

Can some one please write a clear documentation on how to setup new domain with CORS enabled/disabled. I couldn't find any good source to get following working.

How should I add new domain with CORS in application.rb?

How can I differentiate domain cors requests between GET, PUT, POST, DELETE, OPTIONS for new domain added.

When developers work on localhost how can they communicate with GIT server, they keep getting CORS error?

Using OAuth service provided by Gitlab to authenticate user. I keep getting CORS error when I try to Authenticate.

I did changes in NGINX to make this working. Going through bugs I see this can be configured on gitlab server itself. Can someone please create a good material which can be shared among community. If already exist please redirect it from here. I put in all key words for users to navigate in case they need to.