Slice grapefruits and lemons in half and squeeze into a large pot. Take the white ribs and seeds out of the middle and place in a cheese cloth (or clean old white t-shirt). Slice the skins into fine strips and add to the pot.
Add 7 cups of water to the pot. Hang cheese cloth with seeds and membranes over the side of the pot where it is submerged in the water. Add sugar to taste.
Cook over medium/low heat, stirring until sugar is dissolved. Continue cooking until rinds are soft (1-1/2 to 2 hours).
Remove cheesecloth, carefully squeezing out all remaining liquid into the pot. Discard contents of cheesecloth.
Increase heat and bring mixture to boil. When thermometer reaches 220F it is at jelly stage and ready to set (about 20 minutes). Carefully ladle into canning jars. Leave about 1/4 inch head space so expansion won’t break the jar.
Seal jars and put in a water bath. Let jars sit in boiling water for 10 minutes. Remove jars from bath and allow to cool before storing.