Draw some pencil guidelines first matching the screens or photos from original helmets. Starwarshelmets.com is a good resource.
Don't prep your surface with alcohol, use dish detergent, rinse and dry. Watch out for lint.
Use a flat brush no wider than 1/4" and paint up to the pencil lines. Do black first for the front traps, tear ducts and rear vents. Do one coat, then let it dry overnight before you do another coat. Top coating right away will lift the previous coat.
After the black is done then do the gray the same way. 2 coats. Then do the lines with a round brush like a 0 or 00.
Tubes stripes I cut from vinyl and airbrush them but they can be done with a brush if you're careful.
Check my Ata helmet build thread, lots of pics there.