I've found that it is the sound of the scissors that seems to bother some dogs. What I do is "air scissor" around their heads, just rapidly opening and closing the scissors near their heads, while slowly moving the scissors around to top, sides and underneath. In the meantime I am talking to them in soothing tones, or perhaps a morsel of a treat.
I also take the (closed!) scissors and use them to rub and scratch their heads, chins and muzzles again talking and praising.
That's how I train puppies and work with scared older dogs whose heads require a lot of scissoring. First do the "scissor massage", then on to the air scissoring, then I can generally get in a few cuts, with tons of calm quiet praise. That part is important, keep your voice low and calm "what a goooood puppy!", so they don't get all excited and you have to try to get them calmed down and start all over again
