I have done several of these. It takes a wee bit of practice at first, but once you get the hang of how to hold the knife, it is very simple. I like to add poly-fill to select areas to make mine a little more 3 dimensional. It is fun and you can do it while relaxing in front of the TV.
They are so fun to do. The only thing I don't like is trying to find the right thickness foam board to do my own designs. So, I went ahead and ordered a couple of different sizes from them, just to have on hand.