How to make secured login page (https)

I wondering how to serve login page in secured way. By some reasons I can't serve all page by https, so I want to do something like that: My page url is http://www.example.com, but login and register subdomain is https://ssl.example.com

Of course I have ssl certificate, I only don't know how to make this login mechanism.

Example page which work in that way is ebay.com, check out url in homepage and login page.