You'll need at least a 1 liter bottle of vodka - depending on the size of the
watermelon. Then just let it soak for at least one day, two would be best.
Also, and this is important: You're going to have to roll the watermelon
every 2 to 4 hours, to make sure the vodka soaks into the whole melon.
Otherwise, it'll just settle on the bottom. Just roll it over and let it sit.
Hope this gets to you in time.
Good luck!
Happy Holidays!