Add simple syrup according to how sweet your watermelon is as well as how sweet you prefer your cocktails. I add between 1/4 to 1/2 cup. To make simple syrup combine equal parts sugar and water, heat just until sugar dissolves. Remove from heat, cool completely. Store in the refrigerator in a sealed jar for up to 2 weeks.
If you like a little fizz in your cocktails, add in a splash or two of club soda or lemon-lime soda.