How to break apart grooved PCB
Bare hands should be sufficient. Grab the boards solidly and bend. Do so in a consistent fluid motion, and hold them such that the boards themselves won't bend much. You might have to work the seam in the opposite direction a few times to make them separate. 

There are depanelling tools for these sorts of things, in the form of vertical wedge presses, or wheels similar to pizza cutters, but you only really need those for situations where you can't bend without damaging the boards, or when dealing with large quantities.

Doing it by hand is fine.