Bring water and sugar to boil in a small covered saucepan. Stir often.
Boil until sugar dissolves. Stir often.
Remove mixture from heat and add in 10 mint leaves. Use a wooden spoon to muddle (crush) the mint leaves. Return lid and allow syrup to steep until the mixture is cooled to room temperature. This is enough simple syrup to make about six drinks. Store in an airtight container in your refrigerator and this can last for one week.
Make mojito:
Add ice, tequila, syrup mixture and lime juice to a cocktail shaker. Shake the cocktail vigorously until the mixture is cold.
Strain mixture into a glass with ice. Add a mint leaf for garnish (optional).