Place two saucers in the freezer to test the jam later.
Use ripe, jucy and undamaged plums. Wash them thoroughly.
Cut the plums in half and remove the stone/pits. Leave the skin because it has pectin that is useful for the jam to set.
Place the halved/pitted plums in a wide saucepan or pot. Add water. Bring them to a simmer over a low heat. Use a wooden spoon to stir occasionally.
Cover the pot and cook for 20-30 minutes until the plums are soft and start breaking down. The time will depend on the amount of plums and pan.
If you want to have a smooth jam, take the pot off the heat and then blend with a hand blender. Put the pot back on the stove. (I like chunky jam so I did not do this.)
Add the sugar and lemon juice. Keep stirring till the sugar dissolves.
Increase the heat and bring to a rolling boil. Keep stirring. Keep on the rolling boil for about 10 minutes. The plums should have broken down and your jam should be thicker now. Take it off the heat once it looks like it has reached the setting point (this also depends on the amount of plums, the pan and the heat applied - could be 5- 10 minutes)
If there is scum on the jam, add a knob of butter and mix. This will remove the scum.
Take the saucer out of the freezer and put some jam on it. Once it cools down push it gently with your finger. If it wrinkles (even a little bit), the jam is ready. If it doesn't wrinkle and remains liquid, put the pot back on the heat and cook for another 5-10 minutes or so. Check it on the second saucer. You can also use a sugar thermometer to check if the jam is done, which should be at 104C or 220F.
Let it the jam stand in the pan for about 10 minutes to allow the fruit to settle. Then stir the jam, mixing thoroughly and then ladle it into the sterilised jars. It will be hot so be careful.
Close the lids and keep the jars aside for at least 24 hours to allow the jam to set.
Keep the jars in a cool dark place. When you open a jar to use the jam, store it in the refrigerator afterwards.