Start by sterilizing your jars and lids. The easiest way to do this is to pop them in the dishwasher on a hot cycle. Alternatively, you can wash them in hot soapy water, then rinse and dry them thoroughly. Place the jars upside down on a baking tray and put them in a preheated oven at 140C/275F/Gas 1 for about 10 minutes. This will ensure that they are completely clean and free from any bacteria.
Next, prepare your fruit. Wash the oranges and lemon in hot water, then dry them with a clean towel. Cut the oranges in half, then slice them thinly. You can also cut them into quarters if you prefer.
Place the sugar in a large saucepan, then add the oranges and lemon. Stir gently to combine.
Place the saucepan over a low heat and cook slowly until the sugar has dissolved and the fruit is soft. This will take around 45 minutes.
Once the fruit is soft, increase the heat and bring the mixture to a boil. Boil rapidly for 10 minutes, or until the marmalade has thickened slightly.
Remove the pan from the heat.
Carefully spoon the hot marmalade into the sterilized jars, then seal with the lids.
Allow the jars to cool completely, then store in a cool, dark place. The marmalade will keep for up to 6 months.