Step 1, Make sure you can use mod_rewrite
The virtual host you want to apply the redirection on needs permissions to use mod_rewrite to perform the redirection. You will need something along these lines a little bit dependent on how your websites are configured to enable overriding options through .htaccess
<Directory /home/yoursite/www>
Options FollowSymLinks MultiViews
AllowOverride All
Require all granted
Allow from all
</Directory>
Step 2, Applying the redirect
Edit or create the .htaccess file in the web directory for the site, and add the following redirect code
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]