It is impossible to hide the backend and an exercise in futility. There are several plugins like Better WP Security that claim this can be done. In plain english that is bullshit, not possible and completely pointless. Ok so now that that myth/gimmick is dispelled let’s move on to real security measures and truths.
Your WordPress Admin backend area is already protected with authentication. In order to get into your /wp-admin folder someone has to be authenticated to your /wp-admin folder / WP Dashboard. In order to cause a problem for your website that person would need to be an Administrator.
I don’t think there is anymore need to debunk this gimmick so I think what you are really asking is this – how to protect your Login Page.
If you use BPS Pro Login Security and BPS Pro JTC Anti-Spam / Anti-Hacker then your /wp-admin folder and Dashboard area are locked down and impenetrable. All other claims are folly and a waste of time.