As simple as it is to do it makes me sick the amount of times it is done wrong!
Get your grill as hot as you can
Put steak on count to 30 (this should give enough time to seer one side if not count longer)
Flip count to 40 (if you had to count longer on the first side then do so on this side)
TURN THE HEAT DOWN! Mid to a low range.
Wait. Once cooked to the right amount remove and serve
There are all sorts of gadgets to test to see if your steak is done but the best way was and always will be to poke it with a trained finger.
To train yourself you will have to put in some grill time (and you can cheat by using a test steak poke it get to know how it feels then cut it open to see what it really is.)
If your grill has hot spots move the bricks around (or replace them). If it has flare ups clean the stupid thing. If you can’t control the heat with it throw it out and get one that doesn’t fail!
*note if you are cooking with charcoal or on a camp fire this is a little harder to do because you can’t just turn a knob you have to plan the grill heat time harder but worth it.