For each Kg of plums I used 600 g of sugar, (because I like my plum jam a little tart) and one teaspoon of pectin.
Wash the freshly picked fruit and place in a large saucepan with a little water. Bring the fruit to a gentle boil. When the fruit is simmering add the sugar and stir well. After 5 minutes add the pectin. Simmer the jam for about 5 more minutes and then pass through a sieve, discarding the stones.
Place the hot jam in sterilised jars, dried in the oven. At this point you can add to each jar a cinnamon stick (for dark plums) or a long pepper (for yellow plums); or you can wet the inside of the jar with a little grappa or brandy (an old Italian method). Either seal the jars with cellophane covers (available in all supermarkets) and secure with an elastic band, or use capsule lids (I use Quattro Stagioni brand).
If using capsule lids, seal the jars well, place in a pot and cover with water. Bring to the boil and simmer for 20 to 30 minutes, depending on the size of the jars. Let the jars cool down in the pot overnight and when they are cold make sure that the capsule has popped by pressing gently on the lid. Properly sterilised jars will keep for over a year.
I also made plum paste, I just used the jam that was left in the bottom of the pot and cooked it a bit longer. Then I pour it into a square container and let it set hard.