The sugar has to dissolve completely to make a smooth Demerara syrup. You will achieve this by bringing it to a boil. Reduce heat to a simmer immediately after that and stir the syrup until the sugar dissolved completely.
Take it off the heat and let it cool down for a few minutes.
Pour it into an airtight container and put it in the fridge. You can store it there for up to a month.
To extend the life of the syrup, add a splash of high-proof vodka.