There is a kind of master recipe for this - identifying basic technique, quantities of ingredients, cooking time, doneness, etc. But you can get some interesting variations by changing the kind of sugar you use and the kinds of lemons.
This year, I made 2 batches - totaling about 20lbs. The first batch I used all granulated white sugar. For the second batch I used jaggery (Indian raw can sugar) for some of the granulated sugar. The photographs in this posting come from the jaggery-based version. The amounts are all metric.
Meanwhile, add the sugars to the pot and stir until completely dissolved. This is very important. Your marmalade may prematurely crystallize if you do not do this.
Add the pectin to the pot containing the oranges and dissolved sugar. Bring to the boil, stirring frequently. Once the mixture is at the boil, boil for at least 15 minutes. Ideally you want the liquid temperature to rise to 224 degrees (F). Once you are happy with the texture of the marmalade, you can place it into jam jars. However, wait a few minutes after turning the heat off, otherwise the peel will float to the top after the marmalade has been jarred.
Use whatever normal method you employ for sterilizing, heating and sealing jars. If using Ball jars, headroom should be between 1/4 and 1/2"
Allow to cool, and store in a dark place until ready to use. This marmalade will "keep" well for at least a year.