Wash the fruit thoroughly.
Using a vegetable peeler, peel the fruit, including the lemon, then finely julienne the skin. Add to the cooking pot.
Cut the ends off the fruit. Then, working with one piece of fruit at a time, remove the white pith with a paring knife. Hold the fruit over a pot as you do this to catch all the fruit juice.
Chop the fruit. Put the fruit and all the juice into the pot, and add the sugar.
Put the pot on low heat and stir until the sugar dissolves. Then, increase the heat to medium/hot and boil the fruit.
Cook, stirring frequently, for about 45 minutes. You can test if your marmalade is ready by testing it on a plate chilled in the freezer. If it’s firm, it is ready. Note that the marmalade will thicken when chilled.
Prepare the preserving jars by boiling them in a large pot for 10 minutes. Ladle or pour the marmalade into the hot jars. Clean the rims thoroughly and seal them with the lids.