For my son’s 8th birthday, he wanted an R2D2 cake (yes, I know he’s supposed to be blue, but bday boy wanted a Red R2D2 Birthday Cake). I baked 3 cakes in 9-inch round pans and froze them. After the cakes were frozen, I simply staked them and frosted them using white buttercream frosting. I also stuck a dowel through the round cakes, which (1) steadied the cake, and (2) allowed me something to secure the head to the cake.
For the head, I made rice krispy treats and formed them into a bowl until it set. Then I frosted the head with grey buttercream frosting and poked a hole in the bottom for the dowel to fit in. For the arms, I again used rice krispy treats and formed them into an oblong pan. Once it was set, I simply cut it into the shape of the arms and frosted it with white buttercream frosting.
For the shapes on the body and head, I simply rolled out some fondant and cut out the shapes.