Rinse the lemons well and then dry them off with a kitchen towel.
Score the skin into quarters and then peel it off and pop it into the bowl of a food processor and chop as finely or coarsely as you like.
If doing it by hand use a sharp knife and slice the peel to your preferred thickness.
Place all the chopped peel in a large saucepan.
Chop all the lemon pulp into rough chunks removing all the pips, but don't throw the pips out for now just place them to one side.
Once all the flesh has been chopped up and added to the pan along with any juice from the lemons. Take all the pips and place them in the middle of a small square piece of muslin or a fine clean cloth and tie it up securely into a loose bag with string. Then to add this bag to the pan.
Stir well then cover and leave to soak overnight.
Next day bring everything to the boil, slowly over a low to medium heat. Stirring occasionally. This will take about 25 minutes.
Then lower the heat and simmer for about an hour until the peel is quite tender. If you have big chunks of peel this process will take a little longer.
Remove from the heat and allow to cool down a little for 15 minutes.
Strain your fruit and juice through a very fine strainer, I would recommend using a proper jam straining kit, this can take up to an hour for all the juice to strain through. But you can use a sheet of muslin tied over a large bowl as a makeshift strainer.
Fish out the muslin bag of pips and give it a good squeeze over the liquid, you will need the pectin the soaked pips have produced to help set your marmalade.
Pop a few small plates into the freezer and return your pan to a low heat.
Pour in the sugar and stir until the sugar has completely dissolved.
Turn up the heat to maximum and boil until you reach setting point - if you are using a candy thermometer which I highly recommend, you are looking to hit 104C / 220F. Stirring every few minutes Until it comes to a rolling boil, then only stir occasionally.
While your marmalade is coming up to temperature you need to sterilise your jars, the easiest way I find is to wash them in hot soapy water rinse them, then place them in the oven with it warmed up to 140C / 275F / Gas mark 1 and leave them in till I need them.
If you don't have a candy thermometer you will just need to keep testing it, first let it boil for 15 minutes then test it every couple of minutes.
To test it, take it off the heat once it's at the temperature or after 15 minutes of being at full boil and place a small teaspoon amount on one of your small plates from the freezer. Leave it untouched for a minute on the plate then run your finger through it, if it wrinkles and sticks to your finger it is ready to jar. If it doesn't return it to the heat to come back up to temperature 104C / 220F or at least another 5 minutes. Then take it off the heat and test it again.
Once it is at setting point allow it to sit and settle for a few minutes then decant it into your warm jars and seal them. And leave them overnight, then label them.
Kept in a dark cool place unopened this marmalade will keep up to 9 months, once a jar is opened, keep in the fridge and eat within 4 weeks.