To carve a pumpkin in this manner, the first step is removing the rind on the pumpkin; to do this one can use large clay tools or a Dremel with a sanding attachment. (If you are impatient like I am I'd go with a Dremel!)
Once the rind is taken away from the section you would like to carve, use various clay tools to achieve the desired look. I find it easiest to start with the nose and then the eyes, mouth and then cheeks.
This was my first attempt at carving a pumpkin in this manner. I found that the consistency changes quickly, so working quickly is a must!