1. Soak the green beans 2 hours before cooking.
2. Put the beans into the boiling water. Remember keep the fire at medium high.
3. When the soup come to boil again, keep the fire at medium high for sometime. You should not leave the kitchen at this moment because it would overflow.
4. About 20-30 mins, the bean skin will float at the top of the soup, remove them. And then put the pot cover on the pot again, a few minutes later some more skin will come up again, remove....cover... remove... until most of the skin come up and be removed. It will spend you about 30 min to finish the job. Remember not to leave the kitchen otherwise the overflow soup would cause you a lot of trouble.
5. Let the soup be cooked by medium low fire and cook for one more hour (or the thickness you prefer).
6. If you like to add some herb (sau cho - sting grass, sorry I don't have its correct English name) do this 10 min before serving.
7. If you like Hoi Dai (seaweed), soak it for sometime and cut into pieces and put into the pot together with the soaked green beans. (at step 2).
8. Another important tip to 令佢起沙, add sugar when the soup is ready.