Prepare the crab apples: Wash the crab apples thoroughly and trim off any stems or blossom ends. You don't need to peel or core them for jelly, as the pectin is in the skin and seeds.

Simmer the fruit: Place the prepared crab apples in a large stockpot and add the water and lemon juice. Bring to a boil, then reduce heat and simmer, covered, for about 30-40 minutes, or until the apples are very soft.

Strain the fruit: Ladle the cooked apple mixture into a jelly bag set over a large bowl or another pot. Allow the juice to drip through naturally for several hours or overnight. Do not squeeze the bag, as this can make the jelly cloudy.

Measure and cook the juice: Measure the strained juice. For every cup of juice, you will need approximately 3/4 to 1 cup of granulated sugar. Return the measured juice to a clean pot.

Add sugar and boil: Stir in the measured sugar until dissolved. Bring the mixture to a rolling boil over high heat, stirring constantly. Cook for about 10-15 minutes, or until the jelly reaches the setting point.

Test for set: To test for the setting point, drop a small amount of jelly onto a chilled plate. If it wrinkles when pushed with a finger, it's ready. Alternatively, use a candy thermometer; the setting point is around 220°F (104°C).

Skim and fill jars: Remove any foam from the surface of the jelly with a spoon. Carefully ladle the hot jelly into sterilized canning jars, leaving about 1/4 inch of headspace.

Process or store: Wipe the rims of the jars clean and secure the lids. Process in a boiling water bath for 10 minutes (adjust for altitude) or let cool, then store in the refrigerator for immediate use.