How to do Google authentication web api using ASP.NET framework from scratch?

I created an empty web api project (without authentication) and made my controllers and Database.

Now i want to make google authentication, what should i do? All the data online shows how to do google authentication if u created a project with authorization ticked. So can u tell me how to start?

api/Account/ExternalLogins?returnUrl=%2F&generateState=true

private IAuthenticationManager Authentication
{
    get { return Request.GetOwinContext().Authentication; }
}