Log in on the control panel of my web hosting as described in “ How do I obtain access to my web hosting control panel? 

Click on the tile "Domains"

 You will now enter the old control panel. Click on "Hosted domains".
Click on "Add new Subdomain":

Fill in the sub domain name:

You can link Webhosting to your domain name (or you can do this later).
Click "Next".

Click "Finish".
Your subdomain is now created.