The stormtroopers are my favorite (of course)! Using a tube of Wilton Black icing and a No 1 or No. 2 tip, outline the black parts of the cookie. Fill in with a bigger tip (No. 2 or 3), then press smooth with a dry finger. I used white cookie icing for the white parts. For best results, outline with a tube of white icing, the fill with the cookie icing.
For the other cookies, again - the secret is to do a thin outline of the area using a No 1 or 2 tip, then fill in the space. When the icing is set, add details, such as the details on Fett's helmet, Yoda's eyes and ear hair, etc. Darth is the hardest - black on black just doesn't show up that well. I ended up just icing the cookies black and piping details with a No. 2 tip.