Porters or MacIntosh will make a sweet jelly; Gravensteins or Greenings will make a spicier jelly.
You will need enough apples to fill a large kettle. Wipe the apples and remove the stems and the blossom ends and cut in quarters (do not pare—a large amount of the natural pectin is lodged in and just under the skin). Cut crab apples in halves. Put prepared apples in a large stainless steel or enameled kettle. Add cold water to almost cover the fruit—there should be about an inch of apples out of the water, but you should be able to see the water level in the kettle. Cover and cook slowly over low heat until the apples are soft.
Mash the fruit slightly while it is still in the kettle. Suspend over a large bowl a damp jelly bag or colander lined with wet cheesecloth. The juice will drip through the bag or colander into the bowl. Pour the kettle contents, fruit and liquid, into the bag or colander and allow the juice to drip through into the bowl overnight. Meanwhile, do something else so that you will not be tempted to squeeze the bag to hurry things along. Squeezing (or pushing through the colander) will not hurt the flavor of the jelly, but it will cloud it as minute particles of pulp will come through into the juice.
When the juice has dripped through, measure out 4 cups (leave the rest for another batch). Heat the sugar in a double boiler; with 4 cups of juice, you’ll need 3 cups of sugar. Bring the juice to a full rolling boil, then add the heated sugar and bring the mixture back to a full rolling boil. Boil quickly, stirring, until the candy thermometer reads 8 degrees above the boiling point (about 220°F) or until the jelly sheets off the spoon. Skim and pour into jelly glasses and seal.