I have used this technique for so many cakes- replicating a well known figure is hard because it requires exact measurements or the character looks off. I find an outline, coloring page, or black and white photo of the character on-line, enlarge or shrink it to the size I need and then trace it onto wax paper. 
I then start with the big outlines- like the shape of the head, then work to the smaller details like eyes. I place the outline on my rolled fondant, and use my small sharp scissors to cut out the shapes. For these princesses I saved the outline of the head so I could use it as a guide for placing the eyes. I use the cut outs like a puzzle to make sure I am placing my fondant pieces in the right space. Obviously the more details the more time consuming it will be but the end result is so much fun!
Adding the obvious details of the figures like hair (goofy ears, pirate hair) really bring the character to life!
a little faith and a lot of frosting