I first made a basic snowman shape, then began to scrape away from it gradually, sculpting the shapes I wanted. I used a pointy spoon to dig away the excess snow (making gaps between the ribs, eye sockets, etc).
Once the shapes were close to what I wanted, I sprayed some cold water on the snow, using a water bottle with a trigger that sprays a mist. Wetting the snow like this helped it pack together tighter, for more detailed fine-tuning sculpting. It also allowed the snow to look smoother, and when the water froze, it was pretty strong.
This was all done with experimentation, but I bet I could do something really crazy and detailed if I put my mind to it.