You grip the plate on the smooth sides (not by the handle cutouts) while your elbows at the edge of your knees, and basically "fan" yourself only moving your wrists. Your thumbs will be on top, and palms facing each other.
And....
^ nailed it.
I have to use a hammer grip in fact for narrow, or else the pump in my forearms and biceps becomes agonizing. It's silly that my arms think they can overpower my back, but they try if I don't really keep an eye on form.