Remove the stems and the small dried tuft (the beard) from each crab apple, wash the fruit well under cool running water, and discard any fruit with insect damage, bruising, or soft rot.
Cut each crab apple in half. There is no need to peel or core them, because the skins and cores hold much of the natural pectin and color.
Place the halved crab apples into a large heavy-based pot and pour in the 3 cups (720 ml) of water, adding just a little more only if the fruit is not nearly covered. Bring to a boil over high heat, then reduce to a steady simmer over medium heat.
Simmer for 25 to 30 minutes, using a potato masher to crush the fruit against the side of the pot every 10 minutes or so, until the crab apples are completely soft, pulpy, and have collapsed into a rough mash.
Set up your jelly bag on its stand, or line a large colander with several layers of dampened cheesecloth set over a deep bowl. Carefully ladle the hot fruit and its liquid into the bag or lined colander.
Let the juice drip through on its own, ideally overnight or for at least several hours. Do not squeeze or press the bag, no matter how tempting it is, because pressing forces pulp through and turns the finished jelly cloudy instead of clear.
Once dripping has finished, measure the strained juice in a large measuring cup and note the exact volume. Add 1 cup of granulated sugar for every 1 cup of juice you collected.
Pour the measured juice into your clean preserving pan and stir in the 2 tablespoons (30 ml) of lemon juice. If you want a spiced jelly, add the cinnamon stick or the tied bag of cloves now.
Set the pan over low heat and add the measured sugar. Stir gently and constantly until every grain of sugar has fully dissolved. You should not feel any grittiness when you drag the spoon across the bottom. Do not let the mixture boil until the sugar is completely dissolved, or the jelly can turn grainy.
Turn the heat to high and bring the mixture to a full, rolling boil that cannot be stirred down. Boil hard, stirring often so it does not boil over, and clip on your thermometer.
Cook until the jelly reaches 220°F (105°C) at sea level. Above sea level, subtract 1°F for every 500 feet of elevation. If you added spices, remove the cinnamon stick or clove bag once the setting point is reached.
Turn off the heat and spoon a small puddle of jelly onto one of your chilled plates. Wait about 30 seconds, then push a fingertip through the puddle. If the surface wrinkles and holds its line, the jelly is set.
If the jelly floods back into the gap, return the pan to the boil and test again every few minutes.
Once the set is confirmed, skim off any foam or scum from the surface with a slotted spoon to keep the jelly clear. Using a canning funnel and ladle, fill hot sterilized jars, leaving 1/4 inch (5 mm) of headspace at the top.
Wipe the rims clean, apply the lids, and screw the bands on until fingertip tight.
For pantry storage, process the filled jars in a boiling-water canner for 10 minutes, adjusting for your altitude. Remove the jars and let them sit undisturbed for 12 to 24 hours.
Check that each lid has sealed by pressing the center. A sealed lid stays down and does not flex.