Grab two glasses and divide the syrup equally between them. Add ice, then pour in the soda water. Top with 1 tbs half and half each.
Big batch method: If you're looking to make a lot at once, there's a quicker way to do things. Combine the syrup and half in half and let chill. When ready to serve, pour about 3 tbs into a glass, add ice, then top with soda water. It doesn't look as dramatic as the one by one version, but it tastes just as good!