With mine, I aim right between the ears and just behind them. It's easiest to do this from behind the rat.

I just use a sharp downward snap of the wrist, like hitting a drum. Not full force, which would probably kill them, but just a good sharp hit. If I didn't do it quite hard enough the first time, the second does the trick. You'll get a feel for it in no time, unpleasant as that sounds.

Gale