Bought some lemongrass and tried making this at home.
For those who want to try,
1) Wash and cut the lemongrass half and remove the root and any yellow leaves.
2) Thump the lemongrass with the blunt side of the knife to make it easier to release juice when boiling.
3) Put in hot water and bring to boil, then lower the fire and simmer for 1.5 hrs.
4) Add sugar and/or ice to liking.