Takeo himself said on the ESPN Football Today podcast (dated 1/18/2012 at 16:45 min mark), that he started working out his neck in High School after one of his coaches told him you need a strong core and neck to hit harder and avoid getting neck stingers and shoulder problems.
He said he would use a 45 lb plate and lay off the end of a bench and do his neck exercises like that. Basically, it sounds like he would be lying on a bench Face UP with a 45 lb Plate on his forehead and do Neck Resistance workouts. He probably also did this Lying Face Down as well, just with lighter weight as 45 lbs is a lot of weight to support with your neck. If you're just starting out, I'd go with 5 or 10 lbs and try to bang out 15 to 20 reps (you'll feel it).
So that's how Takeo got his crazy side neck muscles.
Source(s):
Lying Face Up Plate Neck Resistance - http://www.bodybuilding.com/exercises/...
Lying Face Down Plate Neck Resistance - http://www.bodybuilding.com/exercises/...