How To Activate MultiSite in Wordpress

WordPress comes with a built-in capability to create multiple websites using the same WordPress Installation.

All you need to do is install and setup WordPress like you normally would either from scratch or from the Plesk panel Applications tab. After that, you just need to enable the multisite feature.

Source: WordPress Codex https://codex.wordpress.org/Create_A_Network

Enabling MultiSite

To enable Multisite, connect to your site using a FTP client or Panel file manager, and open the wp-config.php file for editing.

You need to add the following code to your wp-config.php file just before the / That’s all, stop editing! Happy blogging. / line.
Insert these lines:

 /* Multisite */
 define( 'WP_ALLOW_MULTISITE', true );

Choosing a Domain Structure Multisite

On a WordPress multisite network, you can add new sites using either subdomains or sub-directories.

Sub Domains: http://site1.example.com
Sub Directories: http://example.com/site1/

It is usally easier to use Sub Directoriesas the only thing you'll need to activate is Pertty Permalinks

If you choose Sub Domains, you will need to create those Sub Directories in DNS first, then Enable Wildcard Subdomains in your WordPress network.