Fill a highball or other tall glass with ice. Pour over half of the grenadine (or all, if you want to give it a layered look). Add the lime juice.
Top up the glass with ginger ale, around 7oz/200ml and serve, traditionally garnished with a maraschino cherry (though here I used a fresh cherry).
Notes
I highly recommend using homemade grenadine if you can - it's easy to make, uses real pomegranate juice and is less sweet (and without the high fructose corn syrup) of many bought versions.