1. Select ripe grapefruits: look for heavy fruit with thin, smooth skin and a fragrant aroma. 2. Wash and dry the fruit. Roll each grapefruit on the counter under your palm to loosen juices. 3. Cut each grapefruit in half crosswise. 4. Juice using a manual citrus press/reamer or electric juicer into a bowl or measuring cup. 5. For a smooth, cocktail-ready juice, strain through a fine-mesh sieve into a clean container, pressing solids with a spoon to extract remaining juice. Discard pulp or reserve for use. 6. Taste and, if desired, adjust sweetness with simple syrup (start with 5–10 ml / 1 tsp per cup and add to taste). Optionally, stir in a small pinch of salt to round bitterness. 7. Cool quickly: chill in the fridge or an ice bath before use. 8. Label with date and use within recommended shelf life.