Tuck your chin in to give yourself a double chin and then push your head directly down into the foam pad (or towel etc) and hold a static contraction. Your physio will tell you at what intensity to perform the contraction, how long to hold it, the rest periods between contractions and the reps/sets.

remember to breathe. Relax your shoulders - don't recruit your upper traps or other scapula muscles - just use your neck muscles - you should feel it at the back of your neck