For the lines, draw them in pencil first! First mark a center line down the helmet. Use a ruler to set yourself reference dots down the face, each pair of dots equidistant from the center. Then connect the dots with a pencil. Once you are satisfied with the look, you can paint or etch them.
Honestly, I think the slight indent given by a rotary tool ads a a very nice touch. That being said, you have to make sure you have a thick enough shell to do that, and you have to be twice as careful since any mistakes will be way harder to fix. If you don't trust yourself to do it by hand (I wouldn't trust myself, haha) I would just paint them.