Not sure which method they will use for the anesthesia - IV or mask. If they give it in the IV, she will feel a warm liquid. If they are using a mask, do you have a scuba mask you can demonstrate with? Her arms will get tingly, like when her leg falls asleep. its okay if she laughs. okay if she doesn't. she'll take a short nap, and when she wakes up you and/or a nurse will be waiting for her.
I've had numerous surgeries. When I was 17 I was in pre-op. I was getting my ACH worked on, was going to be in a leg cast to my hip. The girl next to me, also 17y (and could've been my blond twin), both in braids, was having a brain operation and getting her head shaved. Hope they don't mix us up! I know she said, 'I hate crutches!'... My mom then asked what type of anethesia I thought they'd use -meaning IV or mask. I said I don't know. laughing gas? then I started CRACKING UP!!! I was laughing so hard I was shaking the bed and you could hear me throughout the whole pre-op. Other patients started smiling and giggling at me. One dr. said 'who in the world is laughing?'... the anesthesiologist came in and said 'I don't know if she needs anything!' I still smile when I think about it.
As for the pain, just tell her that her neck will be sore for a bit, you'll have medicine that will help her.
If it were me, I would probably have her wear a turtle neck shirt or sweater from now until then so that she can get used to the feel of something on her neck, restricted chin/head movement. Maybe have her wear one of those kids travel pillows around her neck? So that by Tues. she is used to the feeling.