Keep the DB at your side and shrug. Brace your stomach as you are shrugging to get some ab work in at the same time. You can make the movement harder by manipulating the tempo or adding in static pauses.
I usually put these in on dynamic day because the weight used is relatively lighter than a Barbell shrug. This fits into our framework of less intensive movements and more volume on dynamic days. Doing this lets us follow a high/ low sequencing in the training and allows for greater adaptations in the end.
Typically we'll do 4 rounds to failure (20-30 reps). The only rest you should take is switching your hands. Just because you are big doesn't mean you have to be out of shape!