Pick out well-ripened plums. It's best to remove their pits ahead of time and mash them.
Pour the fruits into a large, suitable container. After 3-4 days, add the sugar dissolved in water (1:3 ratio); the water needs to be at a temperature of 77°F (25 °C). If adding 44 lb (20 kg) sugar, you need 16 gal (60 L) of water. Stir until it dissolves and mix with the fruit mash.
Stir this material 3-4 times per day, the more the better. After about 14 days, the fermentation process should be complete and the mixture should have cleared up on top, although that all depends on the temperature, the quality of the sugar and frequency of stirring. Don't close the drum during fermentation. It's best to measure alcohol content with an alcohol meter - it needs to be about 17-18 % ABV. If the alcohol content is higher, add more water, if it's lower - add sugar.
In some regions they like to add a little vinegar (1 bottle per drum) or citric acid if the plums are too overripe. There needs to be about 12″ (30 cm) of empty space between the material and the top of the container, at least during fermentation.