This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
The amount of sugar used should depend on the amount of pectin and acid the fruits contain. As a rule from three-fourths to a pound of sugar is used for each pint of juice. If a sour jelly is desired, of course, not so much sugar should be used as for a sweet jelly. Measure the juice, put into a granite or porcelain-lined kettle and boil for fifteen or twenty minutes, or more according to the amount of water that has been added, and the desired strength of the jelly. In the meantime place the sugar in the oven, and heat until hot, but not until it browns. After putting in the sugar it should be ready to take off in from five to ten minutes; but one should always test jelly by dropping some on a cold saucer or plate. If it does not jelly right never make the mistake of boiling it over again. If you do this you will have a thick, muddy, stringy compound, which is far from the clear, tender yet firm, quivering jelly which should be the result of your work. The best way to do if the juice refuses to jelly properly is to add to it more water, and more fruit, and boil to a thick marmalade or butter.
Fill your jelly into hot glasses or jars that have been thoroughly sterilized. Cover with melted paraffin before placing on the lids. Nowadays one may obtain screw top glasses and jars that are not much more expensive than the ones with plain lids, and the jelly is more likely to keep in these, since they can be sealed tight.
Jams only differ from jelly in the fact that the pulp of the fruit is used, as well as the juices, the fruit being put through a sieve or fruit press.