It's not easy to work the neck unless you have access to well-designed neck machine, which few people do.
Personally I use isometrics. I suspend weights from my forehead with a strap and hold my neck in a flexed position against gravity for as long as possible.
A better strategy, which I have not attempted yet, but have seen done, is to rig a football helmet with a metal pipe welded to a square plate and screwed onto the top of the headgear. You then secure barbell plates to pipe with a collar.