How to create a subsite in Drupal

1. Create a subdirectory inside sites folder of your Drupal installation, it will be inside docroot folder.

   

   Drupal-Multisite Directory Structure

2. Create a virtual host. You can follow any of the steps from traditional way to executing a bash script. I recommend executing a bash script so that you don't need to go through all steps everytime.
3. Make a copy of /docroot/sites/example.sites.php called /docroot/sites/sites.php
4. Add the below line of code inside the sites.php.
   $sites['sub-site-address.domain'] = 'subsite';
5. Check the newly created subsite from your browser 'sub-site-address.domain' and install the site.
6. Create folders for modules, themes, libraries etc., and add your custom theme or module specific to this newly created site if you wish.