H.K.
Wait until she is passed out, or just after a bath,w hen the nails are soft and subtle. This way, there is no real surprise, no pain, and you can make it seems like fun, with faces, gasps and all kinds of fun noises, as you do it.
I hope this helps you. Just make sure you have a firm grip in case she gets excited, or wakes up on you. Be prepared to get child-like, and make it seem like you woke her up for a good thing. You don't want to cut her, so make sure you got the grip, when doing it. Just in case!
Good luck, and best wishes.