To make this Darth Vader cake topper I used the 'Superhero' templates. In the 'TEMPLATES' tab below, you'll find additional templates which will help you create Darth Vader.
HEAD & FACE
Make the head the same way as in the 'Superhero' tutorial, but don't make the chin cutout. Stick the mouth first and then the eyes. 

HELMET
Roll out the paste to about 1-2 mm. Cut out the helmet parts (see templates). For tips on how to roll out the paste so that it doesn’t move when cutting, go to step 131 of the ‘Superhero’ tutorial

First stick the front part of the helmet. 
Stick the back part of the helmet after you attach the head.