Add crushed ice to a glass, enough to fill it about 2/3 of the way.
Crushed ice
Fill the glass most of the way with carbonated water, leaving about an inch at the top (enough space for the flavored syrups or cream, if using).
8 to 12 ounces carbonated water
Add your desired amount of flavored syrup. More syrup will make the drink sweeter, and less will keep it mildly flavored.
2 to 4 tbsp flavored simple syrup
Add an optional splash of cream or half and half, if desired. This will turn the drink into an Italian cream soda, also called a "French soda."
1 to 2 tbsp heavy cream or half and half
Mix the drink up a little to blend all of the flavors together.