Gather all the supplies. Bottled lemon juice would be fine (the fresher the better!) and simple syrup is preferred, but clearly not essential. ;)
Halve the lemon. Slice a lemon wheel to garnish the cocktail. (A twist would also work.)
Juice half the lemon (or add 1 fluid ounce / 30 ml of bottled juice).
Add 3 dashes of Angostura bitters.
Add 2 fluid ounces / 60 ml gin.
Add 1 fluid ounce / 30 ml simple syrup or 3 dashes / tsp sugar. Stir.
Shake with ice or pour over ice and garnish with the wheel.
As with many cocktails a lot of variation is possible. This is mine from 12 Bottle Bar's, in turn inspired by Dale DeGroff's.