The information in this wiki article was originally posted by Nick Perry / Dak on the 501neg forum.
It has been reformatted and annotated to better suit the wiki.
From Nick: “After a good 8-10 hours of bondo and cutting and sanding, I'm feeling pretty good about the helmet.”
The key things required to finish the NEG hybrid kit as an AotC style helmet are:
Fill in 1” of the visor in each corner.
Create recessed circles on the sides, at each ear.
Fill in the frontmost vent slot and cut 2 more at the back.
Drill out the teeth of the frown.
Cut out space to install a red “brake light” on the back of the fin.
When trimming the helmet, make sure to leave a 1/2” neck ring along the bottom edge.
Nick's photos show you the changes very clearly. Note in the last picture that he is making an ARC trooper, so the additional hole on the right ear is for the rangefinder. This is not required for non-ARC clones.