First off, you must be using a real TLD domain to create a tutorial

Make sure you have your domain name added in your YaKontrol panel!

Then select the domain name that you want to make a subdomain for

Go to the "Subdomains" section, then click "Add a new subdomain" next to the little + sign

A little box will slide down, then you enter the name of the subdomain you want to create!

Put the path to the files for that subdomain in the "Document path" box! Also, make sure that directory exist before you create your subdomain, or it probably won't work for awhile.

Click the "Create subdomain" button, and you're subdomain will be created!

It generally takes about 5 minutes for your subdomain to be created, so be patient! If you went to the subdomain before it was actually created and recieved the 404 error page, it might not work for you for awhile since your ISP caches DNS request (it still works for others though).