Boil crab apples in water until soft. Wet a jelly bag and strain crab apples through it. (For clear jelly, do not squeeze bag.) Measure the juice, then simmer in a stainless steel pot for 5 minutes. Skim froth, then add sugar. Simmer for about 10 minutes, until jelly thickens and drops from a spoon in sheets (about 220 degrees F). Skim foam and pour jelly into dry but hot sterilized jars to within ½ inch of top if sealing with paraffin (use ⅛ inch of wax) or to within ⅛ inch of top for two-piece metal lids. Seal, cool, and store in a cool, dark, dry place.