I have only managed to cut one claw too short, and that was cos the cat moved at the wrong time, it only bled for a couple of minutes though, so I suspect I didn't cut that far down!! Susanne - I manage to do the cats back claws, I just hold them like a baby, you do get the occasional one that is a nightmare though, but I resent paying £8.50 at the vets for it to be done.
Lisa, it really is easy, and best if you get them used to being looked at from a young age, better than suddenly starting when they become seniors and their claws are more prone to growing long and thick. And, for the cats that hate the vets, a better option than going there on a regular basis.