Flat Iron only needs to be handled so that it stays juicy and tender and not overcooked. Salt/Pepper or rubs that you find in the recipe section works well. Heat should only be high to sear both sides for no more than 2 minutes per side, after which the heat needs to be lowered to cook the center without overcooking the surface. Since this is such a thin cut, it is suitable for pan frying, but after searing on each side, it needs to be flipped frequently to keep from overcooking, and it does not need much time past the initial searing. It is best to move to a plate to rest for 10 minutes to let if even out the temperature, then it is to let it cook too long in the pan to get the center finished off.
If you get good at learning the pressure test with your finger, this is a great cut for that to work, and to learn it. It cooks fast. Using an internal meat thermometer is a good way to ensure not overcooking it. These devices cost $20 and are critical for the best eating experience and are as useful a tool as a spatula. The internal Fahrenheit temperature goals are: 115-120 Extra-Rare (Bleu); 120-125 Rare; 125-130 Medium Rare; 130-135 Medium. Not recommended to go above 135 degrees.