How did u get it to look like Elmo is popping out?  Elmo is sitting on top of the base cake or is there a cutout in the middle of the base cake to fit Elmo in? 

Farhas, Elmo is placed on top, like a stacked cake. I then attached the curled back pieces around him. Hope that helps!

Trying to make this for my daughters birthday!  What is the sister for the base cake?  And what is the diameter for Elmo?  I was thinking of making a 6 inch diameter Elmo on top of a 12 inch base cake? 

I don't remember the exact sizes here since I made it 6 years ago. I'm guessing I did a 8 or 9" cake and Elmo was 4 or 5", but it sounds like your 12" cake and 6" Elmo will work perfectly.  Good Luck!