Just put cranberries, sugar, and water in a medium saucepan and bring to a simmer over medium heat.
Then lower the heat a bit to medium-low and continue to cook for about 10 minutes, before your berries start bursting. Remove from heat.
Once the berry mixture has cooled, you can strain them. I ended up letting my mixture cool overnight in the fridge. I wanted to make sure the berries had seeped as much as possible and left lots of flavor behind. But I don’t think it was totally necessary. You can strain the berries right away if you prefer.
Now you’re ready to get your drink mixing on! Put about 10 leaves of mint and a T of lime juice in each glass. With the back of a wooden spoon or a muddler mash the mint up.
Now put ice in each glass and pour 2 oz. of the cranberry syrup and 2 oz. rum into each glass.
Top off each glass with soda water. And sprinkle some fresh cranberries in for garnish.