• Install WordPress
• Add the following line just above /* That’s all, stop editing! Happy blogging. */
• define(‘WP_ALLOW_MULTISITE’, true);
• Last step enables the Network Setup item in your Tools menu.
• Go to Dashboard => Tools => Network Setup.
• Do as directed. It will ask you to choose from the following two network options:

• Sub-domains — a sub-domain based network in which sub-sites use subdomains. Example: http://lamp.sangkrit.net
• Sub-directories — a path-based network in which sub-sites use directory path. Example: https://sangkrit.net/lamp

• If you choose sub-domain network option then you will need to generate wild card from your c-panel check this tutorial: Subdomain Network: Adding A Wild-Card DNS In DNS Server
• Network details get filled in automatically but you are free to make changes:

• Server Address: The domain of the URL to access your WordPress installation.
• Network Title: The title of your multisite network.
• Admin E-mail:  Super admin’s email address for whole network.

• Click the Install button.
• Follow on screen instructions.  It provides you with cold blocks to add on your site’s wp-config.php and .htaccess files and prompts you to create a directory:

• Create a directory for media file uploads. Directory should be writable by webserver.
• Add the specified lines to your wp-config.php file
• Add the specified lines to your .htaccess file
  If you do not have a .htaccess file, then create it in root directory.

• Clear your browser’s cache and login to your site.
• From your Dashboard at the top left corner you will see My Sites menu. Now you may visit your Network Administration from My Sites => Network Admin => Dashboard.