Wash and pick over the fruit, checking for any bugs and rotting fruits. Then chuck them in a large, heavy bottomed pan - make sure there is plenty of room in your pan (a preserving pan is not essential here), the fruit ought to only fill it to a third maximum. If you do not heed this advice, you will find as it is cooking it will spit and you will have a very messy cooker etc, and you may well get burnt by the sugary syrup!
You should not need any further liquid, as the fruit will yield plenty. Slowly bring the fruit to a boil, and add the lemon juice. Turn down the heat and simmer for 15 minutes, stirring occasionally with a wooden spoon. Take care to pay attention to the fruit, as it is possible for it to catch on the bottom of the pan, and if it does the jam is ruined. Once you have boiled the fruit, while still simmering, add the sugar and stir until dissolved, once it has all dissolved turn the heat up high and create a rolling boil, skimming off any scum that may form - DO NOT STIR the jam now until it has finished - if you do it takes AGES to come to the right temperature, and I have found it can have a tendency to burn if stirred?! Put your jam thermometer in and wait for the temperature to reach 105c - NO MORE than this, remove from the heat immediately and decant into warm sterilized jars using a ladle and a jam funnel - careful the jam is VERY hot and sticky, it will burn badly. You will need about 5 jam jars - dependant upon the size, with a total capacity of 2kg. Voila!