Option 1: Weight Behind Head

Starting position.  Put weight behind your head.  .

Sit-up with weight.  Keep the weight stationary.  Do not move it in any direction.

Final position.

Option 2: Weight In Front of Chin and Neck

Starting position.  The dumbbell should rest on the top of your shoulders. 

Hold dumbbell in front of you and keep it against your body.

Final position.