To be honest, a video would be the easiest way to tell and give you a clear answer.
All I can add is that when I do them, I hold them at my sides, slightly further back (very slightly) and I pull up and backward as far as I can and flex at the top and hold it.
My gym also only goes up to 95-pound dumbbells, though (the 100's are there, but the rubber coating is off of them and they feel lighter than the 95's, yet some people still use them...different story, though), so I tend to get most of my trap work done in the cage or on the Smith machine.
I actually like doing solo shrugs on the Smith, if you're looking for something different. Stand with the bar parallel to your side and lift up and back with one hand; this is a way you can stack more weight on (unless you're lucky enough to have heavier dumbbells).