Sterilise the jar
Preheat the oven to gas 1, 140°C, fan 120°C. In a bowl of hot soapy water, thoroughly wash a 1-litre clip-top jar with a rubber seal. Place the jar in the oven to dry completely, then boil the rubber seal in boiling water for 2-3 minutes to kill any bacteria (do not place this in the oven as it will damage). When the jar is dry, remove from the oven and allow to cool slightly. While the jar is still warm, sprinkle 2 tsp salt into the bottom – this will keep it sterile.