Oil grill grates before lighting to prevent sticking. Preheat grill to medium-high heat. Sprinkle salt and pepper on both sides of flank steak and allow to sit in the refrigerator for 20 minutes.
Combine ranch seasoning, water, and olive oil. Place flank steak on the grill and baste with ranch mixture, flipping every 5 minutes and continuing to baste.
Grill steak for 20 minutes or until it reaches 145 degrees with a meat thermometer for a tender medium rare, or 165 for firmer well done steak. Allow to rest for 5-10 minutes before slicing across the grain into very thin slices. Enjoy!