Salt your flat iron steaks on both sides and let them sit, uncovered, in the fridge up to overnight, and at least 1 hour. I like to set them on a rack over a tray to catch any juices. This preseasons your meat and will help develop a better crust on the grill.
Get your grill very hot and clean and oil your grates as I described above. When you start this process, take your steaks out of the fridge to warm up a bit. You still want them cool, though, so no more than about 15 minutes.
Grill your flat iron steaks hard on one side, a good 5 minutes without touching them. Flip and grill to the doneness you like, anywhere from 2 to 10 minutes, depending on the heat and the thickness of your steak. Use the finger test for doneness to determine this.
Move your steaks from the grill, coat them with freshly cracked black pepper - or whatever spices and herbs you like - and let it rest 5 minutes. Slice across the grain (the narrow end), and squeeze citrus juice over it all.