It takes time and patience to make marmalade because the orange peel needs to cook for a while to soften and it also takes a lot of time to cut the peel into small strips. But it is well worth the work.
I used about 6 big oranges, I didn't weigh them but when making marmalade its good to use equal parts sugar to fruit.
1 kilo of good quality oranges
1 kilo sugar
1 tsp of Agar agar (or any other thickener)
First juice the oranges and save the liquid for later then scoop out the pulp, this can be cooked with the oranges for flavour.
Place the pulp in a muslin cloth and tie it into a bag with some cotton string. Put the halved peels into a big pan with the pulp bag, and add 1 litre of water. Bring it to the boil and let it simmer for 1 hour until the peels are soft but not too mushy.
Once all the oranges are cut, add them to a big pan together with the juice and the liquid the peel was cooked in. Then add the sugar and heat gently. When the sugar has dissolved turn up the heat and boil for about 10-15 minutes. It is important to do a set test; take the pan off the heat and place a spoonful of marmalade on a cold plate and wait to see if it sets, but if it hasn't, return it to the heat and test again until it does. Let the marmalade sit in the pan for 5 -10 minutes before filling the jars.
I also use this marmalade to make a very nice chocolate orange cake.