3 large, ripe grapefruit 2 1/2 quarts cold water (more as needed) 8 cups sugar (more as needed)
Wash the grapefruit and dry with paper towels. With a knife or vegetable peeler remove the skins without cutting into the white pith. Cut the peel into strips about 1 inch long and 1/8 inch wide. Cut away and discard the white outer pith. Cut the fruit in half crosswise and wrap the halves one at a time in a double thickness of damp cheesecloth and twist the cheesecloth to squeeze the juice into a bowl. Wrap the squeezed pulp in the cheesecloth and tie securely. Add enough cold water to the bowl to make 3-1/2 quarts of liquid.
Drop in the bag of pulp and the strips of peel. Let the mixture stand at room temperature for 12 to 24 hours. Pour the entire contents of the bowl into an 8 to 10 quart stainless steel or enameled pot, and bring to a boil over high heat. Reduce the heat to low and summer uncovered for 2 hours, stirring occasionally.
Discard the bag of pulp and measure the mixture. Add 1 cup of sugar for every cup of mixture, stir thoroughly, and bring to a boil over moderate heat. When the sugar has dissolved increase the heat to high and boil briskly, stirring occasionally, for about 30 minutes until the marmalade reaches the temperature of 220F on a jelly, candy, or frying thermometer.
Remove from the heat and skim off the surface foam with a large spoon. Ladle the marmalade into sterilized jars or jelly glasses. To prevent the peel from floating to the top, gently shake the jars occasionally as they cool.
I need to use more peel, not enough. Ony 1 hr for first cooking; took 1 hr to get to jelly temp. It was otherwise easy to do. Have picture, made 12 jelly jars. Used red grapefruit. Will use 1 extra C of sugar next time.