Spirit + citrus juice + sweetener, shaken/strained into a glass, then topped with soda water
It can be easy to confuse a fizz with a Collins, since for the most part the ingredients are the same. The fizz however has some slight differences. The spirit, sweetener, and citrus should first be shaken with ice before straining it into a glass, then topping it off with soda water, ideally from a charged soda siphon in order to really add a nice bubbly fizz to it.
There is another way in which the fizz differs from the Collins, and that is that there are some additional variations of the fizz. A “Silver Fizz” will include the white of one egg, “Golden Fizz” uses just the yolk, and a “Royal Fizz” uses the whole egg.
For an example, see: