Log into your control panel.
Click on Domain.
Click on Sub Domain Pointing Manager. You’ll get this screen.

1) Enter the new sub-domain name.
2) Select the domain you want to put it on.
3) Enter the internal directory you want to point the sub-domain to. Ok, this is where it might be confusing, but it’s actually pretty cool because you get to do it right here instead of having to create a separate re-direct after you create a sub-domain. Just enter the same name as field #1 and be done with it.

OR…

Sub-domains are nothing more than pointers to an internal folder and those pointers are re-directs. If you type in blog.yourdomain.com, it redirects you to a folder called. yourdomain.com/blog. So if they are the same name, then why make you enter field #3 at all? Because it could be someone to hard to remember or type. Say you have a folder called marys-blog-about-surfing.

So instead of going:
marys-blog-about-surfing.yourdomain.com which would point to yourdomain.com/marys-blog-about-surfing

You’d want to do this:

surfingblog.yourdomain.com and that points to yourdomain.com/marys-blog-about-surfing