Remove pulp from grapefruit shell, slice rind into strips, then slice strips thinly.
Place rind into saucepan and cover with water.
Bring to a boil and simmer until rind is transparent, adding more water if needed.
Measure fruit and liquid, add equal amount of sugar, and simmer until the jelly stage is reached.
Add lemon juice and simmer for another 5 minutes.
Pour into sterilized jars and seal tightly.