Fill a tall glass with ice, then fill it up to ¼-way with flavored simple syrup, such as vanilla or strawberry. (If you don't want it to be too sweet, add a little at first and taste it. You can always add more.)
Top with club soda. Stir gently to combine.
Garnish with fresh fruit, herbs or a straw.
Notes
The beauty of the Italian soda is how versatile it is! Try one of these variations:
French soda: Adding a splash of milk or a bit of heavy cream makes it a French soda, but you could also call this an Italian cream soda recipe.
More/less sweetness: Use more or less syrup when making an Italian soda.
Ice cream float: Add a scoop of ice cream to turn your Italian soda into a float.
Non-carbonated soda: If carbonation bothers you, use distilled water to make non-carbonated flavored water.
Sugar-free Italian soda: Use sugar-free syrups to make your Italian soda.