A really good, easy recipe for a homemade mint sugar syrup is to take four cups of water, one two cups of sugar, and a two cups of loosely packed mint leaves.
The mint leaves must be fresh, so pick them off the stem yourself; the dried ones won't work.
You put your water and sugar together in a bowl, then put it on the stove at a low heat. Keep stirring until the water warms up and the sugar dissolves completely. As it starts to dissolve, add in your mint leaves.
Then turn up the heat and bring the mixture to a rolling boil. Depending on the thickness you want, let it boil for one to two minutes, stirring all the time to keep it from sticking and burning.
Then turn off your stove eye and let the mixture cool down to room temperature. Once it's cool, put the liquid through a strainer to remove the mint leaves, and put the remaining liquid in a clean container.
Glass is best, but plastic will work if you're not planning on storing it for a long time.
This makes an excellent addition to mint juleps, mojitos, and even margaritas.