.htaccess - RewriteBase? Setting up virtual directory roots

I’ve set up my first file and web server and most of my client’s sites have file paths relative to the root. How do I configure each site’s folder on the webserver to act as the root directory of that project? I’ve tried using RewriteBase but it doens’t seem to work (and yes, I’ve turned on and set up mod_rewrite. All of my other rewrites work). Any help is greatly appreciated.