This was my first real success with fondant - it's so fun when it comes out well! I originally was going to make a Mario cake for my son, but he was sold on King Boo.
I found a picture of King Boo online to help me with the face (I also referenced the other Boo example from this site). I used one cake mix (we chose strawberry) then used a 10" springform pan for the body, and a loaf pan for the arms, part of the crown and part of the tongue. I had some extra cupcakes from another cake, so I used 2 - one for the large part of the tongue and one for the center of the crown. You could use another cake mix and just make a square cake for those pieces, but I used what I had.
I pieced the homemade King Boo cake together, doing the crown on a separate plate, then did a crumb coat with buttercream and let dry overnight, uncovered. The next day, I added a thin layer of buttercream to hold the fondant, then used Marshmallow Fondant (recipe at - http://whatscookingamerica.net/PegW/Fondant.htm) to cover the cake. Just one big circle of fondant, then pinching it gently to shape to the cake.
Once I had the fondant trimmed and molded, I gently outlined the face. I used red and black icing tubes to do the face. I put the red icing on with a knife, then smoothed it with the knife dipped in water. Then I outlined the tongue with black and did the eyes. The teeth and eye whites were cut out of fondant.
Lastly, I used colored fondant to cover the crown, then laid it on top of Boo's head. Once it was set, I added the jewels, also made out of fondant.
The homemade King Boo cake was a huge success at the party.