You'll need access to WHM and the ability to edit DNS zones.
1. Go down to "Edit a DNS Zone" under "Dns Functions" and select mywebsite.com.
2. Click the Edit button.
3. Go down to "Add New Entries Below this Line" and put "order" or "order.mywebsite.com." in the first text box. Either one will work, but if you're going to put the full host name, be sure to include the period at the end, otherwise it will think it's a subdomain of "mywebsite.com" instead of the full host name.
4. Click the drop-down list and choose CNAME.
5. Put "www.somedomain.com." in the text box after the drop-down list. Don't forget the period at the end, or just like in #3, it will think it's a subdomain of "mywebsite.com" instead of the full host name.
6. Click the Save button.