The whole preparation process takes most of a week. The last step takes 2 hours. Do not be in a hurry. You do not have to use "real" canning jars; if you save glass jars & lids (from spaghetti sauce, pickles, etc.). Also find TONGS to get the jars out of the hot water.
Wash etrog and trim off the ends. Slice etrog thinly, or chop into very small pieces (food processor).
GET RID OF BITTERNESS:Put the slices and the seeds into a muslin or cheesecloth bag. Day 1: Cover with water and let stand at least 12 hours. Bring to a boil, and simmer uncovered for 20 minutes. Drain fruit. Cover with cold water & repeat on Day 2.
Day 3: Drain fruit, then cover with cold water & simmer uncovered 1 hour. Take out the bag of fruit and seeds. Let it cool down, then squeeze every drop into the pot with 6 cups water and throw out the mush.
Now add sugar and continue to simmer about ½ hour. To become jelly, there has to be the right concentration of sugar, so if you put in more water or less sugar it will just take longer to boil down. It will probably reach 200º quickly, but then it takes up to an hour to get up to “jelly” temperature (220º–222º). After it gets to 215º stir constantly and DO NOT WALK AWAY – it burns easily after this point. If it does burn, do not stir up the burned parts – dump the jelly into a clean bowl, wash out your pot, put the jelly back in and continue.
Put a saucer in the freezer to check for when your jelly is done. It should gel when you pour a little on your very cold saucer. If it still won’t gel after getting to 220º, add 1 package of powdered pectin or ¼ cup of orange marmalade, and cook 15-20 minutes more! If you overshoot the “gel” point, you will end up with more of a candy than a jelly.
How to put in jars: Put your clean jars and lids in a pot, fill the pot and jars with cold water. Bring to a gentle boil. Drain out a hot jar, fill with hot jelly, put on the hot top tight. As it cools, the little button on the top that popped up when you first opened it will pop down again. This is what keeps out the bacteria that would spoil the jelly.
Keeps up to 3 years in pantry. After opening, keep in fridge 4-6 months.