Wash the jars in soapy water, rinse and then place in a warm oven upside down to dry.
Check over the raspberries and remove any with a sign of mould, avoid washing if possible, or at most, rinse gently in a colander.
Place in a large jam pan, or the largest pan you have, add the lemon juice and put over a low heat to start the juices running.
When raspberries are heated through, remove from the heat and stir in the sugar until dissolved.
Place back onto the heat, turn it up higher and bring the contents of the pan up to the boil, then reduce to a 'rolling boil'.
When the jam has darkened and thickened slightly, usually after 6 mins or so, you can test for a set. Turn off the heat.
Ladle into the warm, sterilised jars, seal immediately. The jam will keep, in a cool, dark place for several years - there is no need to refrigerate either open or unopened.