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.