The OAuth section of the ACP only lists fields for the key and secret for each service. It does not tell you where you need to get these from. It also does not tell you about the information you need to provide to those services to make them work. For example, Google OAuth requires that you set up a consent screen, as well as specify the request URI to expect. We will have two separate request URIs that the user needs to provide, one of which includes a link ID which may not be the same:
http://.../ucp.php?mode=login&login=external&oauth_service=google
http://.../ucp.php?i=ucp_auth_link&mode=auth_link&link=1&oauth_service=google
The only way I got these was by looking at the error message that Google gave me claiming that the set up was incorrect when I tried to link the account or log in.