Simmer crab apples in water until the apples are soft, and strain the mixture through a jelly bag, reserving the juice. Measure the juice, add 1 cup sugar for each cup of juice, and cook the mixture over high heat until the jelly thickens and falls off a spoon in a sheet. Pour the jelly into sterilized jars, and process in a hot water bath.
To make crab apple jelly with pectin, roughly chop 3 1/2 pounds crab apples. Place the chopped apples and 3 cups of water in a saucepan, bring the mixture to a boil, and reduce the heat to simmer the apples for five minutes. Strain the mixture through a jelly bag or cheesecloth, reserving the juice.
Combine 5 cups of the juice in a pan with 1/2 teaspoon butter and 7 1/2 cups of sugar, and cook over high heat until the mixture boils. Add a packet of pectin, stirring constantly until the pectin dissolves, and remove from heat. Pour the jelly into sterilized jars, and process the jelly in a hot water bath.
To make flavored crab apple jelly, add 3 to 6 whole cloves to the apples while they are simmering in water, or add 4 to 6 sprigs of mint to the apples while they simmer.