1- First of all, put your all ingredients on the work surface. Then, wash your damson plums. If you have picked them yourself double check of any holes or blemishes and remove the debris and stalks etc. Then, If you are using a knife to remove the seeds (stones), remove them from the plums and cut into halves (or quarters).
2- Then place the damson plums in a medium size pot and add the water. Then, start cooking mixture of Damson plums and water on medium heat and stir with the spoon until the fruit is soft. Keep cooking the jam mixture for about 10 or 15 minutes or until the your damson plums has completely fallen apart.
3- Then pour the marmalade mixture into another pot (or food processor of blender). Then blend marmalade mixture until to creamy puree. Then place your blended damson plums into pot and add granulated sugar over the mixture and mix them. Then cook again the mixture of marmalade for abour 20 minutes. And don't forget to continue the stirring. If it happens, remove the scum or foam from the surface and add a knob of butter and stir for decreasing the scum/foam.
4- Then add lemon salt (or lemon juice) over the mixture and boil for 5 more minutes. Then test this by spooning a little on to a chilled saucer. Cool slightly then push with your finger - if it wrinkles it is ready. If not return to the heat for couple of minutes, and then test again.
5- Then, remove the marmalade pot from the heat and cool for a few minutes, Lastly, pour your marmalade carefully into clean glass jars. Close the lid and let it cool. Then, store in a dark and cool place or store in the refrigerator.