This Marvin the Martian cake I made by using the Wilton 3D teddy bear pan. Bake as directed. I iced it using colored icing. Any details I piped using #3 tip. I found it easier to ice the whole cake using a large bag for each color and only having a hole cut in the tip. I just went back and forth to fill in, then smoothed it out with a small angled spatula. I looked off a photo of Marvin to get it as close as possible.
The brush like thing on his helmet, I made a week in advance using fondant with a tablespoon of gumtex in it. I made the base of the brush, then attached the top with a little water. To make it look like bristles, I took a small paring knife and repeatedly made little slits all along the top. Kind of like scoring it. To secure this to his helmet, I put a dowel through it and about 3″ into his head so it would hold fast.