Slice the lemons thinly, remove the seeds, and put the fruit into a large pan along with the water. Tie the seeds in a small cloth and add to the pan. The seeds will add extra natural pectin. Bring to a boil and then simmer, covered, for 1½ hours.
Strain the juice through a jelly bag. Squeeze the bag of seeds into the jelly bag. Measure the juice and return it