I don't have exact measures for the ingredients, and it really is a matter of trying it and figuring out how you like it. Geno likes A LOT of vanilla flavoring. I like a little more cream. You get the idea!
For your test run, put about 1" of flavored syrup in the glass. There are many flavors out there to try, but I suggest starting with vanilla (since pretty much everyone likes vanilla and you can use it in sodas if you don't care for the creme soda). Also, I would suggest paying a little more for a good quality taste. We bought the brand from Wal-Mart-since we were already there shopping-and it wasn't very good. We also bought the sugar-free French Vanilla from Sam's (a good price!), but we quickly learned that there is a huge difference between Vanilla and French Vanilla...it wasn't very good. I also have raspberry, and I know there is hazelnut. It is all a matter of taste, so make what you like. Next, pour in the sparkling water leaving about 1 1/2" to 2" of space at the top. Obviously, if you are using a shorter glass, you will need to make adjustments accordingly! Now, pour about 1" of heavy cream on top. This part looks pretty cool as the cream will sit on top and slowly sink to the bottom. Make sure you mix it up REALLY WELL before drinking. If you don't (and you use a straw), you will get a very strong vanilla flavor at first, and then just creamy sparkling water at the end. Not so good! ENJOY :o)