Don't tell her too soon. She'll just have more time to worry. Break the news to a 2- or 3-year-old the day before; give older kids a week.
Explain what she'll see, hear, and feel. The doctor will be wearing a mask, for instance, and the operating room will be cold.
Watch your language. Say "make an opening" instead of "cut." And use terms she'll get: A gurney is "a bed with wheels."
Let her know you'll be right there. Most hospitals let parents stay with kids while they get anesthesia and in the recovery room before they wake up. (Check to be sure yours does.)
Read a book. Seeing a familiar character go through the same experience can be a comfort. One of our faves: Franklin Goes to the Hospital ($5; Scholastic).
Calm yourself. Keep breathing, Mom! If you're tense, your child will be, too.