The next day, measure the juice – you will probably have about 1.2 litres, though this will depend on the berries used. For every 600ml juice, allow 450g sugar. Put the juice into a large pan and bring slowly to the boil. Add the sugar as it just comes to the boil and keep stirring until the sugar has dissolved. Boil rapidly, without stirring, for 9–10 mins until setting point is reached. Test this by dropping a little jam onto a cold saucer. Allow to cool for a min then push gently with your fingertip. If it has formed a skin and crinkles a little, it’s set.