As I recall, last time I just clamped the nut in some needle-nose vise grips, then set one edge in the hole and used a small screwdriver to press the other edge into place.
But then I found some nylon things at FLAPS that push in from the front and lock as the screw goes through. Much better, IMO, so that's what I use now. They are totally hidden once the headlight is installed.