Traditionalists insert a bottle, the fastidious a funnel, and I know one guy who marks out a grid with a surgical hammer to determine the proper places to plunge a turkey needle.
He’s my best friend.
Most any colorless liquor will do. I’ve never spiked a melon with gin, but it might be wonderful. I recommend cutting a fist-sized crater about a quarter way into the flesh, pouring the booze in a cup at a time, and refilling it as the liquor is absorbed. A Charleston Gray takes all day, a moon-and-stars about four hours.