It's one part Grenadine, two parts lime juice, and four parts applejack. If you can't find applejack, you can use calvados, but by no means should you use store-bought grenadine when it's so easy to make your own. Just start with a pint of pomegranate juice, simmer it with a cup of cane sugar till it's reduced to about a cup, and chill it. You can add a bit of 100-proof vodka to it as a preservative, but it'll keep fairly well in the refrigerator without it.
Anyway: shake it well in a standard cocktail shaker (I use my standard 128-count), and serve it straight up in chilled stemware. It's really quite a nice little tipple - not too sweet, just a little tart, and who doesn't like apples?