Peel the grapefruits, removing as much of the white part as possible – this would make your liqueur bitter.
Put the grapefruit peels in a jar with 700 ml – 3 cups of the alcohol. Close it with a tight lid and keep it in a cool, dark place for a minimum of 10 days to a maximum of 30 days.
When the time has passed, put the sugar in a pot with the water and heat it on the fire just until the sugar has dissolved. Cool down completely.
Add this sugar syrup to the grapefruit peel and alcohol mixture (which will be yellow by now).
Add also the remaining alcohol. Mix well and close the jar tightly again. Keep it in a cool, dark place for a minimum of 7 days to a maximum of 30 days.
Filter it and bottle it. Keep it in the fridge and serve chilled, after meals.