1 part lemons: pulp, juice and peel
--- I used 450 g of lemon pulp & juice, plus 50 g pomelo rind,
--- because the lemon peels were not suitable for consumption]
1 part water
1 part sugar
seeds, membranes etc
Cut lemons into slices. Cut slices into pieces, remove the seeds and also the membranes, if possible, and reserve both. While cutting, save the juice.
Measure pulp, juice and peels, put into a large sauce pan. Add the same amount of water. Put all seeds an membranes into a cheesecloth and place it in the pot. Bring to a boil at moderate heat, cover and simmer for about 30 minutes until the peels are soft.
Turn off the heat. Remove the cheesecloth bag from the pot and let it cool until you can touch it. Wring the bag thoroughly to extract the liquid which is containing pectin. Add liquid to the lemon mixture. Add sugar, stir well.
Heat the mixture again and bring to a boil. Let boil briskly until ready (*). That will take at least 10 minutes, depending on the amount: the more marmalade, the more time. (Don't process much more than 1 kg of fruits at a time.)
Once it is ready, pour the hot marmalade into jars and seal.
(*) Take a look into the pot: If kind of "roads" or grooves occur while stirring, which slowly close again, it's time for a jelling test. Put a bit of marmalade on a cold plate. It should get a skin and wrinkle up when you push it with your finger. Otherwise cook a few minutes more.