Here are the instructions the gal who made this Barbie cake sent me (unfortunately this was a long time ago and I don't have her name to give proper credit.)
Of course I will try to tell you how to do it; just bear with me and let me know if you have questions as I may leave things out with realizing. I think these are quite simple to make, really. Start with any regular size barbie doll, a Wilton Wonder Mold pan and a 9"x3" round pan.
Day1: Bake the cakes. The wonder mold takes more batter than the instructions say and usually comes up a bit short, so you really need a 3" high pan for the 9" round. I use so many different recipes that I'm not sure how much batter to tell you for the 2 pans. If you are using box mixes, I would say probably 5 and use the leftover for cupcakes.
Cool the cakes and refrigerate overnight.
Strip your Barbie . Color fondant or gumpaste, roll out a small amount and cut a strip as wide as the length of the doll's torso and long enough to wrap around her; probably about 6"x3". This is the bodice of the dress. Wrap it around the doll and use a paring knife to cut off extra at the back. Once you have the right shape and size, brush a little water on the piece and wrap it on again to stick. Mold it to the doll with your fingers and polish gently with fingertips. The seam down her back should be pretty even so be gentle. Set her aside somewhere to dry overnight.
Day 2: Take the tube out of the wonder mold cake. Stack the 2 cakes and carve the sides with a good bread knife to a more even slope. Mostly, you will be carving the bottom cake at a slight angle, don't carve straight down, you want it to look like a natural curve. When you are happy with the shape, separate the cakes and cut into layers. I cut the bottom cake into 2 and the wonder mold into 3. Fill and stack all of the layers, then ice the whole thing with buttercream. Use the hole from the wonder mold tube as a starting point and, carefully cut around it and straight down through the middle of the cake to make it about a 1 1/2" wide at the top; she has wide hips . Scoop out the cake from the hole. Put it on whatever base/plate/cake drum you want to use and refrigerate for at least 30 min.
Roll out enough fondant to cover the cake. Roll it as round as you can; you can trim to a circle if necessary. Thin out the edges of the circle somewhat, so it will look more like fabric at the bottom of the dress. Find the middle of the circle and make a small hole or mark. Place the circle over the cake with the hole in the fondant on top of the one in the cake. Smooth out the areas you want and fold the areas you want to make it look like a dress with folds and ripples. What is great about these is that it doesn't have to be perfectly smooth, you want the little folds going down the sides. Also, having been in the fridg, the buttercream will have set up and if you need to pull a side of fondant up and rearrange you can without making a huge mess. When you are happy with the skirt's shape you are almost done!
If you made a hole in the fondant to mark the middle, cut it a little more and push it down into the hole in the middle of the cake. Get Barbie and wrap her legs together in platsic tightly. Hold her by the neck or shoulders and gently push her down into the cake. There will be a gap between her waist and the cake so make a sash out of fondant or gumpaste to cover.
Brush the entire dress with Super Pearl Luster Dust- dry brush.
I used small edible pearls as finishing touches and attached them with royal icing. I made her wrap with gumpaste, since it dries harder and will hold the free form shape. I put coarse crystal sugar in her hair to look like pink crystals. These are fun, there are a lot of options when you think about it, especially with the hair and accessories. Check out the websites with collectible Barbies for dress ideas. That's it