You need the following ingredients:
Gordon's Gin, vodka, Kina Lillet and lemon peel.
Preparation, according to Ian Fleming and James Bond:
"Three measures of Gordon's, one of vodka, half a measure of Kina Lillet. Shake it very well until it's ice-cold, then add a large thin slice of lemon peel. Got it?" -Casino Royale, Chapter 7