Wash the oranges and place them whole in a large pot, cover with water and bring to the boil (more water may be needed as stated depending on the size of your oranges and your pot). Simmer for 90 minutes (skins should be soft and easily pierced with a sharp knife).
Remove oranges from liquid and cool (keep the liquid). When cook enough to handle with your hands, cut in half and scoop out the pulp and place into a colander. Press down hard with the back of a spoon and add this to the water.
Slice the orange skins (thick or thin, you choose) and add to the pot. Stir in the sugar and place back on the heat. Stir until the sugar dissolves. Boil over a high heat for 15-20 minutes. Skim off any scum or froth that rises to the surface.
Test to see that the marmalade is set (a small spoonful on a fridge cold plate, it should "set" within 60 seconds). If not, cook some more and re-try for setting again.
Leave hot marmalade in the pot for 30 minutes and stir often, this stops the strips of peel don't sink to the bottom of the pot/jar.
Ladle into steralised jars. Marmalade will keep for 6 months in the pantry OR 2 weeks in the fridge once opened.