Just add the pumpkin to the dry cake mix, and don't add anything else. Its similar to adding applesauce instead of eggs and oil. The pumpkin takes the place of all the other ingredients.
Pour into cupcake liners in the muffin pans and bake as directed. Check for doneness.

Someone that was going to weight watchers gave me this "recipe". It cuts the fat out and adds fiber to the cupcakes, and you don't taste it at all.