To set the trick up, remove the tip of a pencil and lodge it between one of your nails. It won't hurt at all, but will turn your finger into a pencil.
With a short notepad in your hand, ask the spectator to think of a name, any name.
As she thinks about it without telling you, pretend to "read her mind." Pretend to scribble something down in your pad. In actuality, you're writing nothing at all.
Ask her to reveal the name she thought of.
As she does so, act incredulous to the name she mentioned. In actuality, you are stalling.
The reason for your stalling is that you are maintaining eye contact with her to keep her away from your hand as your finger-pencil writes the name she just mentioned.
When done, turn the pad over and show her you read her mind. She'll be flabbergasted.