I've made this several times with good luck. Don't overcook it, the
recipe is rather vague. No more than twenty-thirty minutes, I would
think.

This recipe is taken from:
"The New Illustrated Universal Reference Book"
[London: Odhams Press, Ltd. 1933]

Ingredients:
4 grapefruit
6 oranges
6 lemons
Sugar***

*** Sizes or weights are not provided other than the simple
instruction to add sugar at the rate of 150% of the weight of the
mixture. This ratio is VERY important.

Initial Preparation:
Cook the grapefruit and the oranges separately, and in sufficient
water to cover them, until they are soft enough to be easily pierced
with a fine skewer.

Leave them overnight in the water in which they have been boiled.

Final Preparation:
Next morning, cut the grapefruits in halves. scoop out the pulp, and
rub it through a sieve to remove the pips and rough core.

Shred the rind with a sharp knife (well, you wouldn't use a dull one,
would you?)

Cut the oranges into thin slices being careful to save all the juice.
Add to the grapefruit.

Weight the mixture at this point and add 150% it's weight in sugar
(recipe says, "good sugar" ).

Add the strained juice of the lemons, and boil all together until the
marmalade shows signs of setting when a small amount of it is tested
on a plate. (no time is given, so it may be trial and error at first)

Only the juice of the lemons is used in this particular recipe.

Have at least eight 16 oz. jars ready, fill and allow to cool.

Enjoy!