I make a mint syrup from a recipe in “Savannah Style” — a Junior League cookbook from that fine Southern City. This recipe is used as the basis for mint juleps. But I can’;t see why it wouldn;t work in other applications.
You need 4 cups water, 3 cups granulated sugar and “leaves from 17 to 20 long mint stalks” (I use about 4 cups of lightly packed leaves).
Bring the sugar and water to a boil and simmer several minutes so sugar is completely dissolved, then stir in mint. Remove from heat, cover, and let steep at room temperature for at least 8 hours (I usually go 24 ).