You need to  have the access level to do it, either by rank or by subscription.  Then you just click on one of the stars.

It starts out as citizen, once you've registered.  Then through participation with the site (either by submitting skins or by helping people on the forums) you may be promoted.  The next step after citizen is apprentice. 

If you subscribe to the site then you have more access and may rate.

If you are a subscriber then you need to cross-reference your Stardock account with the Wincustomize account.  Then you will be able to access those controls.

Submitting skins is a good step along the path to becoming an apprentice, but it isn't necessarily automatic.