Start by removing your steak from its packaging and drying it VERY thoroughly with paper towels. You can use washable towels if you'd prefer, but I'd recommend washing them right after to avoid cross-contamination. I use bamboo paper towels for drying raw meats, because they are more sustainable, but also still disposable.
Generously add freshly ground black pepper to one side of the steak. I like to add the pepper first, because it has a harder time sticking to the steak than the salt does, and adding it first gives it a little bit of an advantage in that regard.
Generously kosher salt (more generously for thicker cuts than for thinner ones), sprinkling it from about a foot or so above the meat so that it cascades down evenly, and takes on the appearance of snow.
Grab the steak with one hand (so that the other hand remains clean for salting and peppering the other side), and firmly press the sides of the steak down against the excess salt and pepper, mopping it up as you go so that the sides get well seasoned.
Flip the steak over and repeat the peppering and salting on this side. Let the meat sit at room temperature and season for at least an hour per inch of thickness (you can do this up to 24 hours ahead of time and leave the steak to "dry age" in the fridge overnight on a small rack or plate then bring it back to room temperature before cooking). This will ensure that your meat is well-seasoned throughout the entire steak, not just the surface.
Heat your grill up before putting the meat on it- it's important to get your grill quite hot if you want to get a nice sear on your meat.
Once the grill is hot, place the steak onto the grates and cover the lid of the grill. Cook for about 2-3 minutes, or until the steak freely moves off of the grates (if it's sticking to the grates, that means that it isn't done cooking).
Flip the steak and cook on this side for about 2 minutes for medium-rare. Add about a minute or two for each increase in temperature (for example, cook an additional 2 minutes for medium, 4 minutes for medium-well, etc.) OR for an even more reliable way of gauging doneness, use the hand test. To do this, lightly pinch your index finger and thumb together. Using your other hand, press the tongs you're using against the muscle to the side of your thumb (toward the palm of your hand)- when you press against it, it will have the same amount of give as a medium-rare steak. For rare, simply open your hand and press the same muscle. For medium, lightly pinch your middle finger and thumb together. For medium-well, lightly pinch your ring finger and thumb together. And for well-done, lightly pinch your pinky finger and thumb together.
Once your steak is finished cooking, remove it from the grill and place it on a cutting board with a juice well or wells. Place the butter on top and let it rest for 10 minutes so that it reabsorbs some of the juices that it has released.
You can either slice the steak on the carving board and then transfer it to a serving plate, or transfer it whole. Pour the rest of the juices back over the steak before serving.