Wash jars & lids in hot, soapy water, rinse, then place on a baking tray and put in a low oven for 10 mins or until completely dry. If you're using rubber/plastic seals, remove the seals and cover in just-boiled water. Ensure you also sterilise any funnels, ladles and spoons that you’ll be using.
Put the halved damsons into a jam pan or a large, wide, heavy-based saucepan. Slowly bring to the boil, stirring occasionally to stop any skin sticking to the bottom. Put a couple of saucers into the freezer. Lower the heat and simmer for 15 minutes, or until the fruit has softened.
Add the sugar and stir over a very low heat until the sugar has completely dissolved, up to 10 mins. This step is vital; if you don’t dissolve the sugar, the bottom of the pan may catch and burn and affect the final taste of the jam. Raise the heat, bring to a full rolling boil, then rapidly boil for 10 minutes. Don’t stir until the setting point of 105ºC is reached. If you don’t have a thermometer, test the jam by spooning a little onto a cold saucer (that you've put in the freezer). Wait a few seconds, then push the jam with your fingertip. If it wrinkles, the jam is ready. If not, cook for a few more minutes and test again, with another cold saucer. Once you've reached the 105ºC or setting point, stir the jam thoroughly.
Remove from the heat, skim off any excess scum. Leave for about 15 minutes to allow the fruit to settle; if you decant the jam too soon, all the fruit will sink to the bottom. Pour into the sterilised jars, label and seal.
I left the fruit stones in the jam when I decanted it into the jars. If you'd prefer to have jam without stones, you can remove them before cooking or by putting the just cooked jam through a sieve before decanting.Personally, I don't mind a few stones in my jars. De-stoning the fruit is before cooking is time-consuming (especially if you're preparing many kilos). Alternatively, sieving it while still piping hot can be dangerous.