I made this High School Musical cake recently for my daughter’s 6th birthday. I started with a half sheet for the base and made a one layer (use a box mix for cupcakes) 8 inch round. The megaphone was made from the smaller cake and I iced it in red then made the dimension with black icing.
Around the edge I placed pictures of the characters that came in a bag of table confetti. I just turned it into cake decorations! The rest was just done with icing.
I do this as a business from my house and have a learned some tricks. One of them is when making red, black, or any other deep color icing, make it ahead of time and let it sit, even overnight. This will allow the color to deepen to what you want.