The 2:1:1 ratio (spirit:sweet:sour) is the skeleton key to cocktail construction

Two parts base spirit, one part sweet element (simple syrup, liqueur), one part sour element (citrus juice). This ratio produces a balanced cocktail every time. Margarita: 2 tequila, 1 Cointreau, 1 lime. Whiskey sour: 2 bourbon, 1 simple, 1 lemon. Daiquiri: 2 rum, 1 simple, 1 lime. Once you internalize this ratio, you can construct original cocktails by substituting components rather than memorizing recipes.