Soak in water over night and rince well then
1,slice to stone ,not to remove just a split
2, weigh the plums or damsons
3, add same weight of sugar to sauspan
4, boil sugar til all melted
5,add plums
6,boil till all stones at top
7,when thickens put some on a cold plate from fridgr leave cool if its thick enough pot while warm if not boil more rapid and repeat process till as thick as you want
it some times wont go a thick a shop jam straight away but after a while it will be fine
Or just Google plum jam and follow on screen its easy we made some other day it great