How to use external login provider with a custom user store?

I am developing an ASP.NET MVC 5 Web application that allows user to authenticate using an external provider.

In Internet there are a lot of information teaching how to enable external login providers, but none teaches how to actually authenticate the user in the system when login returns to the MVC site. All information I have found assumes developer is using the default ASP.NET identity user store, so it tells nothing about what to do next.

For example, in Google, when user selects the Google account, the call returns to the site and run the method FindAsync of the custom user store, but, after that, how can I actually log the user in?

This is one my attempts to retrieve the user e-mail:

    public Task<T> FindAsync(UserLoginInfo login)
    {
        var info = AuthenticationManager.GetExternalLoginInfoAsync().Result;
        var emailClaim = externalIdentity.Result.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Email);
    }

But GetExternalLoginInfoAsync does not belong to AuthenticationManager object.

I have read that Microsoft.AspNet.Identity.Owin should be included, but in my case, that NuGet package is already included, and of course, the corresponding using statement is present in the code.

Any help, please?