The easiest way is to go to http://yourdomain.com/cpanel
Replace yourdomain.com with your actual domain tied to the hosting plan you want access to.
You can also get into cPanel directly from the StableHost's Client Area by following these steps:
Log in to the Client Area.
Go to Services -> My Services:
Look for the Web Hosting plan you wish to get into and click on the button called "Active" on the right side of it:
On the left side column click on "Login to cPanel":
* If you want to recover the login credentials of the cPanel account, read this article.