Color a large portion of fondant with blue food coloring gel to make 2 shades of blue: a medium blue and a light blue.
Using a fondant rolling pin or regular rolling pin, roll out the fondant into a thin, even layer large enough to cover the front half of the skirt with the dark blue and the back half of the skirt with the light blue. Cut the fondant as needed with a sharp knife.
Gently apply the fondant to the surface of the cake, pressing carefully to adhere the fondant to the cake through the buttercream.
Roll out a small piece of medium blue fondant to cover the doll's torso. Cut moon shapes of the lighter fondant for the little flaps on the side of the dress that come out from each side of her waist. Roll a thin piece of medium blue fondant into a long snake and then form into a bow to place at the back of her waist. Do the same with the light blue fondant to make straps on her dress. Customize the dress to your desired vision.
Using small dots of royal icing, adhere the gum paste flower and royal icing flowers to the skirt of the dress, along her waist, and along the neckline of the dress.
To finish, remove the waxed paper strips from the bottom of the cake and discard. Pipe a line of buttercream around the bottom of the dress where it meets the cake plate. Stick gum paste flowers in the buttercream to decorate. Use a small food grade paint brush to apply edible shimmer dust all over the fondant to add some sparkle.
Store cake in refrigerator until a few hours before serving. Allow to come to room temperature before serving so the buttercream is soft and creamy.