WordPress Multisite (WordPress MU) is a WordPress installation that can host multiple WordPress sites on one installation. Typically, you would setup a new installation as a WordPress MU install, but it is possible to convert an existing WordPress site into a WordPress Multisite.

If you wish to convert your standalone WordPress site to a WordPress Multisite, follow these steps.

Edit wp-config.php

First, you'll need to edit wp-config.php. Open this file either though the cPanel File Manager, Virtualmin file manager, or however you edit your files on your sever.

Add the following line near the bottom of the file:

define( 'WP_ALLOW_MULTISITE', true );

Your file should look like this, although the line numbers may not match:

Save this file, but keep it open as you will need to add additional code to this file later. If you downloaded the file, upload it back to your server.

Deactivate Plugins and Create Network

Login in to your WordPress site and deactivate all active plugins. This step is critical as it is required in order to setup the network.

Once all active plugins are deactivated, go to Tools > Network Setup

On existing WordPress sites, your only option will be to use subdomains for all of your network sites instead of sub-folders (subdomain is: example.yourdomain.com; subfolder is: yourdomain.com/example).

Give your network a name and admin email. Click on Install. Once installed, you'll be given some code to update wp-config.php and .htaccess files with.

Edit wp-config.php and .htaccess

Open up your wp-config.php again. Add the code given to you by WordPress for the wp-config.php to the file, but add it above the line that reads /* That's all, stop editing! Happy publishing. */ like in the following screenshot. Keep in mind the line numbers may be different.

Save the file.

Open your .htaccess file and find the following lines:

Keep in mind your line numbers may be different.

Replace all those lines with the code provided by WordPress, so the section looks like this:

Keep in mind that your line numbers may be different. Save the file.

Next Steps - Adding subdomains & network sites

Next, you will need to log back into WordPress. Once logged back in, you should see a new option in the top menu called My Sites. From here, you will see Network Admin and your current WordPress site.

In your current WordPress site, you can re-enable your plugins that you had enabled.

To create a network site, go to where your DNS is hosted. Whether it's cPanel, Virtualmin, or Cloudflare. Create a new CNAME subdomain for the network site you want to create. The CNAME value should be your main website.

Once your DNS record is created, go back to WordPress and go to Network Admin and click on Sites.

From the Sites page, you can click on New Site and then fill out the details for the new site:

Themes & Plugins

If you want to install new themes and plugins, you'll need to do it from the Network Admin Themes and Plugins. If you enable any plugins or themes here, you will force them on all network sites. This is useful if you want to keep a consistent theme between your network sites.