The technique for Elmo was to tape parchment paper to a printed image and then trace the outlines with black piping gel. Then, I flipped the parchment paper onto the iced cake and filled in the rest of the details with colored icing and the Wilton decorating tips. I suppose it would not have been that hard to freehand.
The cake was King Arthur Flour's Golden Vanilla Cake, and the icing was all purchased ready-made. I love the instructions at King Arthur Flour, so that's one of the first places I look for basic recipes. The instructions allow a switch between weight and volume, which is also so helpful.
Looks like it was a hit with the birthday boy.