Synopsis:
Your main website Drupal installation is in /home/[user]/public_html) and you want to create www.mainwebsite.com/subsite
ln -s /home/[user]/public_html /home/[user]/public_html/subsitemkdir /home/[user]/public_html/sites/mainwebsite.com.subsitesettings.php to /home/[user]/public_html/sites/mainwebsite.com.subsiteCreate a new database, have the connection information ready and browse to http://www.mainwebsite.com/subsite
Thanks to this reply I finally have my subsite running.