Pare off the rind thinly and cut into shreds. Remove the white pith (white spongy part) from the fruit and cut the pulp (fruit) into small pieces, removing the pips (seeds). Set the pith and pips aside.
Place the water in a medium pot. Add the fruit and the rind.
Tie the pith and pips in a piece of muslin and put the muslin-wrapped parts in the pot of water to soak with the fruit and rind overnight.
The next day, place the pot on a hot stove, boil the mixture gently until the rind is tender. It takes at least an hour to cook it to reduce it by about one third in volume. (Verify that the rind is tender as it will toughen up if you add the sugar too soon.)
Remove the muslin bag of pips, squeezing out the juice.
Add the sugar, stir until thoroughly dissolved.
Bring the mixture to a rapid boil over high heat. After it gets boiling, turn the temperature to medium high. It will continue to boil without requiring stirring and should not boil over. Note: Do not stir "as this encouages the sugar to crystalize," according to Larousse Gastonomique.
Continue to boil the mixture rapidly until setting point is reached (about 20-30 minutes). If you are using a candy thermometer, it will set at about 220F degrees. To check that it is set, place a dab of the mixture on a cool plate and wait a minute or so. If your finger makes a wrinkle when you push the dab, the marmalade is set.
Pour while still warm into clean, warm jars that you've sterilized in the oven, or that are still hot from the dishwasher.
Cover marmalade with a disk of grease-proof paper (wax paper or parchment paper), then cover the jar top with cling film, and screw on the lid. Once the lid is on, the marmalade will keep for years, literally, in your cabinet.
Label and store in the cupboard for your delight. Refrigerate after opening.
Makes about 7 cups of marmalade. If you put the marmalade in half-pint jelly jars, you will get 7 jars. Each serving is about 2 teaspoons (if you are American...and 1 teaspoon if you are British. Ha. Ha.)
https://recipeideashop.com/lemon-marmalade/
Notes
Note 1: January is the best month for citrus fruit.Note 2—Rinse and boil the muslin after use; dry and store in tea towels drawer to use again.Note 3—These fruits are also good for making marmalade: