This is the easy part — let the watermelon sit in the fridge, vodka intact, for 24 to 48 hours. I didn't notice a huge difference in the amount of vodka absorbed after 24 hours, so it's up to you. After it's done chilling, slice it open and serve immediately! The longer it sits, the more vodka you lose since it'll drip out. As you can see in this photo, those skewered holes make a big difference, so don't skip that step.