My own experience on cutting bb's fingernail.
When my bb is awake, I had difficulty cutting her nail. But when she is sound asleep, wow, no problem and you can take your own sweet time to cut them.
Usually I don't cut until very very short (just make sure it is not long to scratch their face will do), I just keep a lookout everytime I check her hands, when the nails are long, then I will cut. Troublesome cos may have to cut often, but for someone like me who is afraid to accidentally cut her finger, this is good.