Many do it at home with an unsatisfied result, especially Chinese from Viet Nam, they always think that only Vietnamese restaurants can do it the best.
Here are a few steps and ingredients needed.
1/ Use yellow crystal rock sugar instead of sugar.
2/ In order to infuse the crystal rock sugar, hot water can make it fast that shall go chemistry with garlic or juice of garlic.
3/ It is suggested to use very limited water (only the said hot water) but juice of lemon.
If you want to be certified, call me when you prepare a Vietnamese meal, I come over.
PS: This is a French woman's recipes.