Mix the sugar and water in a medium saucepan. Heat on medium high and bring to a boil.
Cut the limes in half and give the mint leaves a crush in your hands and add to the sugar and water mixture.
Turn heat down to medium and let simmer for five minutes to steep the flavor out of the mint and limes.
Pour syrup through a strainer and chill.
Keep the syrup refrigerated.
To use this syrup to make a Mojito, pour about 1 tablespoon (give or take) over ice, add your rum and club soda, mix well and serve in a chilled glass.
Now I'm not giving anything exact here because I have always eyeballed it when I make mojitos. Just don't use too much syrup, you should be able to figure out a good ratio to fit your taste after a few tries.