Before you start, sterilise 5 old jam jars (see tip below).
Put the summer fruits in a large mixing bowl and use a potato masher to crush them.
Fold in the jam sugar and lemon juice and stir until well combined.
Pour the jam into the sterilized jars, put on the lids and stand at room temperature for 24 hours.
Freeze for up to 1 year. Be sure to remove the jam from the freezer at least 1 hour before you want to use it. The defrosted jam will keep in the fridge for up to 3 weeks.
Cook's tip: To sterilise old jars, preheat the oven to 140°C, fan 120°C, gas mark 1. Wash the jars in warm soapy water and rinse. Place upside down on a baking tray and warm in the oven for at least half an hour.