If you create a website for a domain using cPanel, as recommended in the totrial video 0.d, the cPanel Addon Domain should be defined as following
Domain = mysite.com
Document root = domains/mysite.com/public_html

Based on this cPanel definition, in JMS, you need to create the slave site using
Site ID = mysite.com
Domain = www.{site_id} , {site_id}
Deploy folder = /home/ACCOUNT/domains/{site_id}/public_html
Where the /home/ACCOUNT is the value of your primary domain that you can find in the "Addon Domain". This is the value associated to your home icon.

In the template folder, you have to put the {deploy_dir}/templates
because you have specified a "deploy folder".

Don't forget in the JMS Templates rule to specify that the action = "copy" for the /templates directory. Otherwise, you will get a symbolic link and you will share the /templates.

You can verify the structure of the /templates directory.
When using "special copy", the subdirectory of the /templates directory are symbolic link.
When using "copy", the subdirectory are a physical copy.

Concerning the "blank screen", perhaps this is due to a wrong directory path that you provided in your cPanel. You can use the "hello.php" (video 7) to verify the cPanel setting. This will also display you the value that must be provided in the JMS "deploy folder".
If the hello.php can be executed and that you have correct symbolic links deployed in your website directory, you can repeat the operation with a hello.php inside the "administrator" directory to verify that your sever follow the symbolic link.
If you get an error to execute the "hello.php" inside the "administrator" directory, you have to check your server configuration to verify that "Options +FollowSymLinks" is active.