1. Gather your ingredients and start by combining the chopped oranges, lemon, and grapefruits with 1 liter (4 cups) of water in a large pot. Place it over high heat and bring the mixture to a boil. Once boiling, reduce the heat to medium-low and simmer for 20-25 minutes, or until the fruit is soft and easily pierced with a fork.

2. Take the pot off the heat and let it cool slightly, about 5 minutes. This is important to prevent scorching or burns when blending the hot mixture. Use an immersion blender to puree the mixture right in the pot until smooth. Alternatively, transfer the mixture to a blender and blend until smooth, then return it to the pot.

3. With the pot back over high heat, add 1 kilogram (2.2 pounds) of granulated sugar. Bring the mixture to a boil, stirring occasionally, until the sugar has dissolved completely. You'll know it's dissolved when you no longer see any sugar crystals.

4. Reduce the heat to medium-low, about 160°C (325°F), and simmer for 10-15 minutes or until the marmalade has thickened. Check for doneness by performing the "wrinkle test": place a small spoonful of the marmalade on a chilled plate and let it cool for a minute. If the marmalade wrinkles when pushed with your finger, it's ready.

5. Once the marmalade has cooled slightly, transfer it to a clean glass jar with a tight-fitting lid. Store it in the refrigerator at 4°C (39°F) for up to 6 months or in the freezer at -18°C (0°F) for up to a year.