Wash the oranges very well, pat them dry and juice them using a citrus juicer. Retain the skins, seeds and the pith (white fibrous part)You should get about 2 cups of juice.

Scoop out the pith using a grapefruit spoon or a melon baller and place it along with the seeds on a muslin cloth. Tie the ends of the cloth to form a potli (pouch). We need to use this when the peels cook as it contains natural pectin which helps the marmalade (or any jam) thicken.

Use a sharp knife to slice the orange skins into thin ribbons (as thin/fine as you can). The easiest way to do this is to place the peels on a chopping board and drawing vertical lines as thinly as possible along them. You should get approx 4 cups of chopped peel.

Prepare the bottles for canning - wash with soap and rinse. If you are going to sterilize them in water, bring a large pot of water to a boil. Place a metal stand/mesh at the bottom of the pan and place the bottles over the stand (but submerged in water completely) and bring to a boil for about 10 minutes. You can also let them stand (washed and patted dry) in an oven at 100 C (200F) for about 10 mins. The lids need to be placed in boiling water for 5 mins, then patted dry with a clean cloth before using.

Transfer the peels, orange juice and 4 cups of water to a large, heavy based saucepan/pan/kadai. Place the pectin pouch (which contains the pith and seeds) - it needs to submerge completely into the mixture to help release the pectin. See notes

Bring the mixture to a boil, reduce the heat to a medium and cook until the peels soften and look transparent. This could take anywhere between 30-45 minutes depending on the thickness of the peels.

Once the peels have softened take the pan off the heat. Remove the pectin pouch and let it cool for a bit. When it is cool enough to handle squeeze it firmly to extract as much pectin as possible - add this to the pan of cooked peels. Once done, discard the pectin pouch.

Place the pan back on the heat. Now you need to add the sugar but the quantity is an approximation. The general rule of thumb is to use equal quantities of fruit peel after it is softened (you need to measure the fruit peel mixture after cooking it for 30-45mins) and add equal amounts of the sugar. Again, if the fruit used is really sweet you may want to use lesser sugar. Also, the mixture when hot will taste less sweet so reserve some sugar and add about 80% of it. Lastly, if you prefer your marmalade to be less sweet and more bitter you need to eyeball the amount of sugar used so use your judgement.

Cook the mixture with the sugar in it over a medium-low heat till it reaches jam consistency (see notes for how to use the candy thermometer). Now, the mixture is going to take a while before it reaches the desired consistency. You need to make sure that you are watchful and don't let the mixture burn or caramelize (which will happen very quickly once the jam stage has been crossed if you neglect it). The steel plates kept in the fridge will help you check the consistency from time to time. When the mixture thickens to the consistency of thin honey it is almost ready. Just put a drop on a chilled steel plate and let it cool a bit (blow air over it rapidly) before pushing it with your thumb. If the drop of marmalade turns wrinkly, it is ready. If it spreads and looks more liquidy, you need to cook it for some more time. You can also draw a line over the drop of jam (like slicing it). If it comes together quickly, making the line disappear, the mixture is still loose and needs more cooking. If the line remains as it is, your marmalade is almost ready. see notes

Quickly spoon the hot marmalade into the sterilized jars, quickly clean up and spills and dribbles along the mouth of the jar using a clean tissue and fasten the lids (when the mixture is still hot). When the mixture cools down the pressure inside will form a natural seal making the lid pop when you try to open it.

If prepared hygienically the marmalade won't spoil for months when stored in a clean dry place. However, for best results, refrigerate once opened.