Wash green plums carefully, remove the stems and drain off.
Poke holes lightly all over the plums with a bamboo skewer.
Put the plums in a pan without overlapping, pour in enough water to cover the plums, and heat over low heat.
When the water in step 3 reaches about 50degrees Celsius (about the level where you feel hot when you put your hand in the water), discard the water.
Repeat 4 three times to remove the scum.
Dissolve 600 ml of water and sugar in a saucepan to make syrup.
Add 5 to 6, cover with a cooking sheet, and simmer over low heat for about 30 minutes.
When the heat is removed, transfer the plums to a container and fill it with the cooking liquid to the brim and store it.