Grapefruit and elderflower complement each other very well, and make a refreshing marmalade. Pink grapefruit tends to be sweeter than yellow grapefruit, but you can use either in this recipe.
Halve the grapefruits and lemons and squeeze out the juice and pips. Tie the pips, and any extra membrane that has come away during squeezing, in a piece of muslin/cheesecloth.
Either by hand or using the shredding attachment of a food processor, thinly slice the grapefruit and lemon peel, with its pith, into shreds.