All jams are made in roughly the same way; use the same amount of sugar as fruit , soften the fruit, then add the sugar, boil, and pour into jars, putting the lids on while hot.
In more detail for plum jam:
1. Stone your plums, and cut them into approximate quarters
2. Add a dribble of water to your plums, and the juice of a lemon if you have it (this helps with setting, which is not usually an issue with plums, as they are naturally high in pectin)
3. Weigh your plum mixture (write this number down), and transfer it to a LARGE saucepan ( you'd be amazed how much they can rise up the sides when you boil them and you really don't want plum and sugar overflow all over your stove!)
4. Soften the plums on the stove - they won't really soften further once you have added the sugar.
5. Weigh out the same amount of sugar as the weight of the plums.
6. Once the plums are soft and in small enough pieces (I sometimes help them along with a masher), pour in your sugar, mix it until dissolved, and wait for the mixture to come to the boil.
7. Boil rapidly for around 10-15 mins - the time will be longer, the more plums you are using.
8. Test to see whether it has reached the 'setting point' by dropping some onto a saucer and leaving it for a few minutes. If it wrinkles up when you push it with your finger, it should set nicely. If not, boil for a bit longer and try again.
9. Pour from the pan into a large jug, and from the jug into jars - right up to the top so there is very little air left in.
10. Immediately screw the lids on and put to one side. YUM!
These make fabulous presents, particularly in small jars ( no-one wants a litre of jam!), and are a taste of late summer when you open a jar in winter, and have a weather-defying cream tea as the evenings are drawing in.
It's worth saying that under-ripe fruit has the highest levels of pectin, so if you can get to your plum tree before the plums are falling off, your jam will set better.