All you really need to know is that sometime toward the end of World War I, somebody in Paris or England put some Cognac, orange liqueur, and lemon juice into a shaker with ice and it stuck. It became popular in both countries, but each made it slightly differently.
French bartenders mixed with equal parts Cognac, orange liqueur, and lemon juice, while the English preferred a manlier two parts Cognac to one part each of Cointreau and lemon. The English version has become the modern standard.
2 oz brandy
1 oz Cointreau (or other orange liqueur)
1 oz lemon juice
Combine ingredients in shaker with ice, shake, and strain into coupe.