The original mixture, according to The Fine Art of Mixing Drinks, is:

8 parts Cognac or Armagnac
2 parts lemon juice
1 part Cointreau or triple sec

Modern mixtures often have equal parts, I prefer roughly 3:1:1

Mix the ingredients in a shaker half full of ice. Strain and serve in a sugar-rimmed glass. Garnish with a strip of lemon rind