Rinse the crabapples, and remove stems, leaves and any bad spots. Leave them whole if small, or halve larger ones. There is no need to peel or core them; the skins and cores hold much of the pectin.

Put the crabapples in a large pot and add water until they are just covered. Bring to a boil, then simmer, partly covered, for 20 to 25 minutes, until soft. Gently mash them halfway through.

Pour the fruit and liquid into a damp jelly bag or a colander lined with 3 or 4 layers of damp cheesecloth, set over a bowl. Let it drip for at least 4 hours or overnight. Do not squeeze the bag, or the jelly will be cloudy.

Measure the juice. You should have about 4 cups. For every 1 cup of juice, measure 3/4 cup sugar.

Put a few small plates in the freezer. Prepare a boiling water canner and wash jars and lids. Sterilize the jars by boiling them for 10 minutes.

Pour the juice and lemon juice into a wide pot and bring to a boil. Stir in the sugar until dissolved, then boil hard, skimming off foam, until the jelly reaches 220°F (104°C), or 8°F above the boiling point of water at your altitude. This usually takes 10 to 20 minutes.

Check the set: a little jelly on a frozen plate should wrinkle when pushed after 1 minute, or the jelly should drop off a metal spoon in a single sheet.

Ladle the hot jelly into the hot sterilized jars, leaving 1/4 inch headspace. Wipe the rims, add lids and bands, and process in a boiling water bath for 5 minutes (at up to 1,000 ft altitude). Let the jars cool undisturbed for 12 to 24 hours and check the seals.