We have a site which is HTTP, but it has an admin part. How can we secure the admin part with HTTPS?
How to do this?
I'll assume you already have an SSL certificate, and have Apache (or $HTTPD) configured properly to work with SSL requests. If this is a bad assumption, let me know.
Otherwise, the quickest way to wrangle WordPress into forcing SSL for logins/admin pages would be the Admin SSL WP Plugin. Install it, activate it, et voilá.
A generic non-application way to force an entire directory and its subdirectories to use SSL can be acheived with Apache:
Simply put that in a the .htaccess file of the directory you wish to secure. If you put it in your root directory it will force your whole site to be secure.