Preparation

1. Backup your database and files.
2. Check if Pretty Permalinks are working on your site
3. Deactivate all active plugins ( you can reactivate plugins after setting up the multisite)

Configure Multi Site

Add the following line of code in wp-config.php, just before the comment
/* That's all, stop editing! Happy blogging. */

/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );

Installing a Network

After the last step you should now see under tools ‘Network set up’
Remember to turn off all your plugins first, you can turn them back on once the multisite network has been created.

Fill in the appropriate details in the asked fields and copy information into index.php and .htaccess files as specified.

Click the ‘Install network’ button

Setting up a new site

After copying the relevant information you have successfully installed a Multisite. Now log back into WP-Admin, and take note that your header bar will display a different set of menu items.

Now you can see the list of multisite under the WordPress installation.
Click on Network Admin > Sites > Add Site, to add a new website to your network.