Using Coda Smultron or an application that allows you to view save and edit invisible files.
Create a file called .htaccess at the root level of the website. With the following content:
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://[domain-name]/$1 [R L]
*Double-check that Allow Overrides is selected for the site in Server Admin to allow .htaccess files to work.