I do tons of wrestlers bridges for the back of my neck. For the front of my neck I put the upper part of my forehead on the ground then roll my head back in forth. Move in all directions when doing these because punches come from all angles. If you train on hard floors, fold up your shirt or towel and put your head on that. Use hands for support if your neck isn't strong enough to support your weight alone.
Dago has great advise and you can also spend 50 bucks on a neck harness. If your neck is untrained you might need to use a harness to work up to a full wrestler's bridge. Neck harness use to be standard gym equipment at any commercial gym but has gone out of style, it is still a great piece of gear though.