Technique
Shake hard with cold ice until the tin is frosted, then strain into a well-chilled glass. Taste before shaking when possible: orange liqueurs differ in sweetness, so an especially dry one may benefit from a small bar spoon of syrup.
A sidecar is a cocktail traditionally made with Cognac, orange liqueur, and lemon juice.

TonicDB editorial guide
The Sidecar is a brandy sour in which Cognac, orange liqueur, and lemon must stay distinct. The 2:1:¾ ratio makes a firm, citrus-led drink; a sugar rim is optional seasoning, not a substitute for balancing the liquid in the shaker.
Shake hard with cold ice until the tin is frosted, then strain into a well-chilled glass. Taste before shaking when possible: orange liqueurs differ in sweetness, so an especially dry one may benefit from a small bar spoon of syrup.
Use a coupe or small cocktail glass. If adding sugar, coat only half the rim so each sip can be taken with or without it. An orange twist adds aroma without introducing more sweetness.
A thick full sugar crust can dominate the Cognac and make a balanced drink seem sticky. Bottled lemon juice or an oversized warm glass creates a flat result, and under-shaking leaves the strong base insufficiently diluted.
For a rounder Sidecar, reduce lemon to half an ounce; for a sharper one, keep the listed three-quarter ounce and omit the sugared rim. Make one adjustment at a time so the Cognac remains the reference point.
These notes explain how to execute and adapt the recipe shown above. Ingredients and serving traditions vary, so taste and adjust thoughtfully.
Contributed by @alfg