My daughter wanted a princess cake for her third birthday. I admit to getting ideas from this wonderful site and others decorating site and sort of combining them how to my taste and needs.
I used an oven proof Pyrex bowl and one round cake pan. This took one cake mix. I saved out one cup of cake batter and colored it pink and used it on the top of the bowl cake, swirling it with a knife so it would be marbled with pink when we cut the dress open.
After the cakes cooled I used white icing to stack them and covered the entire cake in a thin layer of white icing. Then I undress a new barbie doll, saving her clothes to give to my daughter after he cake was cut. Wiped the doll down with warm soapy water and dried her off thoroughly. I centered her on the cake and pushed her down all the way.
This cake took 3-4 cups of icing, tinted pink and a number 16 star tip. I then just decorated it with stars working in a spiral pattern and then created her dress bodice on the doll. I used Wilton ready to use icing in the squeeze bottle to make the white swags and number three on the dress and pink decorators gel add the highlights on the bodice. I added some store bought icing roses around the waist of the dress, just attaching them with decorator gel.
My daughter LOVED her princess and everyone was so impressed that I made her myself!