Place the orange slices in a pot and pour sugar over the oranges.
Combine everything well so that the sugar melts and the fruit slices are covered with it.
Optional: Keep to macerate overnight. This can result in a better marmalade.
Keep your marmalade over a medium to high flame and stir occasionally. Bring to a rolling boil.
Cook down the marmalade over low to medium heat and stir occasionally to make sure nothing sticks to the bottom.
Reduce marmalade to the desired consistency with a hand blender.
The marmalade is ready when the fruit skin appears translucent and cooked. Test if the marmalade is set by dropping some of it into an ice-cold plate. If the marmalade is not running, it is set. You can test setting with a thermometer too (*see post above for details). If it runs, keep it for some more time on the heat and repeat the test until it doesn't run anymore.
Pour the preserve into clean sterilized jars up to the rim.
Pour a few drops of liquor (vodka, rum,...) into the lid to kill the remaining bacteria. Close jam with lid tight. Turn upside down quickly to create a vacuum.
Clean jar on the outside with sponge and label with marmalade name and date.
Store in a cool and dry place until further use. Keep in the fridge once opened.
Organic unwaxed Oranges are a must because you use the skin in the marmalade as well and you don't want odd unknown pesticides in your marmalade
Use a few drops of rum/vodka and similar to drop into the cap. This is to disinfect further (to avoid mold growth in the long run)