So: the trick is, you make parallel slices about 1/4 to 1/2 way through the meat, going *across* the grain of the meat. You can cook it like that, but I usually slice again perpendicularly to the first slices, making "X" shaped slices all along the piece of meat. The flank steak can be kind of tough because of the grain of the meat, but making the slices before you cook it make it easier to eat later.
So then you put as much garlic as you want in the slice-marks (I usually just bash two cloves of garlic and then break them up into little chunks that fit into the slice-marks) If the garlic is nice and deep into the slices then it won't burn, if it's just on top of the meat it gets all charred.
Then you put the steak into the broiler, and depending on how cooked you want it, in about 7-10 minutes it's done. I add salt or herbs afterwards.