Wash the Seville oranges and lemon thoroughly to clean their skins.

Halve the Seville oranges and squeeze out their juice; reserve the juice.

Scoop the orange pulp and seeds from the halved orange shells.

Thinly slice the emptied orange peels into fine strips.

Cut the lemon into chunks and remove all visible lemon seeds.

Place the orange pulp, orange seeds, and lemon seeds into the reusable cloth bag.

Add the sliced orange peels, reserved orange juice, and lemon chunks to a large pot.

Pour in enough water to completely cover the fruit mixture, about 1.4 l (6 cups).

Submerge the cloth bag in the pot and bring the mixture to a boil over the stovetop.

Boil the fruit mixture for 30 to 40 minutes until the orange peels soften.

Remove the cloth bag from the pot and measure the weight of the remaining orange mixture.

Return the orange mixture to the pot and stir in caster sugar equal to about 60% to 70% of its measured weight.

Squeeze at least 1 tbsp of pectin-rich liquid from the cloth bag into the pot.

Add a pinch of ground cinnamon and stir until evenly combined.

Cook over medium heat for 30 to 40 minutes, stirring constantly to prevent scorching.

Stop cooking when the marmalade has reduced and looks glossy; it will firm as it cools.

Ladle the hot marmalade into clean glass jars and allow it to cool before sealing.