If you already have WordPress installed and wish to convert it to a Multisite, you must add the following line to the website’s wp-config.php file via SSH, FTP, or File Manager in Cpanel:
right before the line:
/* That's all, stop editing! Happy publishing. */
Save the change and log in to your WordPress administrator area.
Go to Tools > Network Setup.
Here you should decide on how your future subsites will be set:
set them as subdomains, for example, sub.yourdomain.com.
set them in subfolders, for example, yourdomain.com/subsite
This change will affect all your websites in your network and it cannot be changed after that. Once you are ready, click on Install.
When the installation is completed, you will be provided with two additional codes which should be set in your .htaccess and wp-config.php files.
When you are ready, log in again to your admin Dashboard.
That’s it. You now have a WordPress Multisite installed on your domain name.