Pour sugar into a larger pot and add water. Bring to boil on medium. Immediately bring the temperature down to lowest, and simmer for 30 minutes. Stir occasionally with a wooden spoon.
Take pot off the stove and add the citric acid in. Stir again.
Wash mint leaves and (optional) cut them into smaller pieces. Put them into the syrup, and put the lid on the pot. Leave out for 24 hours.
The following day, heat oven to 250F. Take two 1-liter glass bottles and wash in soapy water. Rinse, pat dry and line on a thin pan. Place in the oven for 15 minutes. This will sterilize them.
Take the bottles out carefully. Transfer syrup into bottles through a strainer or a gauze. Keep the syrup in a cold and dark place for up to a few weeks.
When ready to serve, add 3-4 tablespoons to a cup and top with mineral or regular water.
Notes
Adjust the amount of sugar to taste. Add a little more citric powder for a stronger flavor.