Start by thoroughly washing the grapefruit and lemon under running water. Cut the grapefruit into quarters, removing any seeds, then slice each quarter into thin strips. Zest the lemon before juicing it, ensuring you capture all the fragrant oils from the peel.

In a large bowl, combine the sliced grapefruit, lemon zest, lemon juice, and water. Allow this mixture to soak overnight; this step helps soften the fruit and enhances its flavor.

The next day, transfer the soaked mixture into a large pot over medium heat. Bring it to a gentle boil, stirring occasionally to prevent sticking, until the grapefruit becomes tender.

Once the fruit is cooked down, gradually stir in the sugar until completely dissolved. Increase heat and bring it back to a rolling boil for about 40 minutes, stirring frequently until it reaches a soft set consistency.

To check if your marmalade is ready, place a spoonful on a cold plate and let it sit for a minute. Run your finger through it; if it holds its shape without running back together, it's done.

Finally, pour the hot marmalade into sterilized jars while still hot. Seal them immediately and let them cool completely at room temperature before storing them in your pantry or refrigerator.