You only have to click activate, and in most cases, your site is ready for SSL. So what happens in the background?
On activation, Really Simple SSL:
- Checks for SSL before activating
- Selects the right kind of .htaccess redirect rule
- If needed, adds a load balancer fix to wp-config: if the WordPress function is_SSL() fails, an edit has to be made in the wp-config.php to convince WordPress your website is on SSL.
- If needed, adds a server variable fix to wp-config.php: if your server is configured not to pass any server variable informing WordPress we are on SSL, but the plugin detects SSL, an edit will be made to the wp-config.php to convince WordPress your site is on SSL.
- Changes your siteurl and homeurl to https, and if needed also changes any defines in the wp-config.php.
- Checks for insecure content on the frontend, and replaces to https.
On a multisite installation this works roughly the same, but a bit more complicated, to allow for per site activation on subdomain and domain mapping installs.