Cut the lemongrass stalks in half lengthwise, and pound a little with a heavy mallet or meat tenderizer until they're fragrant.
Place all ingredients in a medium saucepan, and bring to a boil over medium-high heat. Let the sugar dissolve, and boil for one minute. Take off of the heat. Let steep for 20 minutes and strain out the solids.
The syrup will keep in the fridge in a tightly covered jar for a few weeks (if it lasts that long).
Notes
The servings will vary depending on how much syrup you put in your club soda. Start with 1 tablespoon and work up from there to see how sweet you like it. And, it may differ on different days.