You'll need about 1.5-2 litres of watermelon chunks. Start by cutting your watermelon in half and then place it, cut-side down on the cutting board. Trim off the rind and discard. Then cut your watermelon into chunks.
Line a cookie sheet with silpat or parchment and place the watermelon chunks on it. Freeze until solid.
Place the frozen watermelon chunks in your blender jar, and then top with about an equal amount of cold rosé. Now blend.