How to Setup A WordPress Multisite Network

If you are not using a fresh WordPress install for this, then firstly you would have to deactivate all your plugins.
Step 1 Open the wp-config.php file and add the following line to activate MultiSite Network option.

define('WP_ALLOW_MULTISITE', true);

Before the next step, we need to know the difference between a Sub-domain Multisite and a sub-directory Multisite.

Sub-domain multisite: Example: http://subsite.example.com/
Sub-directory multisite: Example: http://example.com/site
Step 2 Now login to your WordPress admin area and navigate to

'Tools' -> 'Network Setup'

and fill in the form. After filling the form you will be directed to a page which asks you to update your 'wp-config.php' file with the settings provided there and also shows the Apache Settings to be updated.

Now paste the codes provided for the wp-config.php file just above the line

"/* That's all, stop editing! Happy blogging. */"

Here is a sample code for Sub-directory Multisite for domain with name example.com

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
$base = '/';
define('DOMAIN_CURRENT_SITE', 'example.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

Here is a sample code for Sub-domain Multisite for domain with name example.com

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', true);
$base = '/';
define('DOMAIN_CURRENT_SITE', 'mu.rtcamp.net');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

Now add the code provided for the .htaccess file.

If you are using Nginx instead of Apache, you can check this tutorial : HERE