Version #1) Carefully mix just before serving, adding the schnapps to the soda then stirring gently to mix well, or the fizz will dissipate too soon.
You can also find butterscotch flavoring near the vanilla flavoring in the baking section of the grocery store, but it is more difficult to find, and actually the flavoring is 35% alcohol where the schnapps is only 15% alcohol by volume, so if you're making large quantities of butterbeer, I recommend just to buy the schnapps. There is not much alcohol content in the butterbeer mixture, just enough to make a house-elf tipsy and to give it the warm, buttery aftertaste to the fizzy cream soda. Yum! :-9
(I did make a refill batch during the party using 1 cup schnapps to a whole 2 liter bottle of cream soda, which was ratio of 1 to 8, but people tasted the difference and complained, so fair warning!)