The following rules can be added to your sites .htaccess file to force visitors to use SSL/HTTPS when visiting your website.

To force SSL on all websites located under your .htaccess, add this rule:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

To force SSL on a single/specific website and not all websites, add this rule:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain\.com [OR,NC]
RewriteCond %{HTTP_HOST} ^www\.domain\.com [NC]
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

In the above example you would replace "domain.com" with your specific domain name.

Please note that in order for SSL/HTTPS to work you must first have an SSL certificate install on your site.