From the St. Germain website, just adding here for reference since the other recipe in the app has no quantities or directions. Very refreshing summer drink.
2 oz vodka or gin
1oz St. Germain
1/2 oz lime juice (~1/2 large lime)
Add all ingredients to a shaker, fill with ice, shake, strain, and serve in martini glass with a lime twist. I prefer vodka over gin, the additional notes in gin are a little odd (to me) in this cocktail.