Weigh the oranges and make a note of the weight, as this will tell you how much of the other ingredients you’ll need, to ensure the marmalade sets well. My basic ratio is: both the sugar and the strained liquid from the sliced and simmered oranges should roughly equal twice the weight of the uncooked whole oranges. So if you start with 500g/1lb 2 oz whole oranges, after cooking you want to be left with a litre/1¾ pint of liquid once the peels have been strained out, and you need 1kg/2lb 4 oz of sugar.
Cut the oranges in half and squeeze out the juice, as this makes chopping the peel less messy. Remove any pips from the juice, spoon any remaining pips out of the peel, place them in a tea cup and cover with water. Then chop the peel into shreds about 0.25cm/⅛in across and place these in a bowl with the juice and cover with water. Leave both overnight, as this will help the marmalade set well.
The next day, place the peel and water in a saucepan. Sieve the pips, place their soaking water (which will have jellied slightly) into the saucepan, then tie the pips in muslin and drop this into the pot. Bring to the boil then simmer for 2-3 hours, topping up with water so that the fruit stays well covered, until the peel is soft when squished between your fingers. Alternatively, cook in a pressure cooker for about 30 minutes.
Strain the juice from the peel and measure it. Whatever the original weight of fruit was at the beginning, you want about double that in cooking liquid. So if you started with 500g/1lb 2 oz fruit, then try and have roughly a litre/1¾ pint (in other words 1000ml or 1000g) of cooking liquid left. If you have more, boil it down in a saucepan to intensify it. If you have less, top it up with water to dilute it.
Then add the sugar (double the weight of the oranges), plus 1-2 tbsp brown sugar if you like to make the colour darker. Add the strained peel, plus 50ml/2 fl oz of lemon juice for every 500g/1lb 2 oz uncooked whole oranges used. Bring to the boil, skim off any white froth and pips that rise to the surface, and then boil until the temperature reaches 105C/220F.
Meanwhile sterilise enough jars in the oven and have the lids washed and ready. When the marmalade reaches 105C/220F, turn the heat off and leave for 10 minutes. Ladle the marmalade into a jug then pour this carefully into the jars, leaving just a bare 0.5cm/¼in gap at the top. Screw the lid on tightly and leave undisturbed until completely cold.