Lets get crackin on some glass shall we ?
You'll need a pin for this one, if this was an MP Prime I'd use something bigger like a boxcutter or Exacto blade.
It's almost as easy as it looks:
Start by scratching a small circle as the center point then scribe in some long outward lines from the center of the circle.
Start in the center and slowly scribe your way outwards connecting the lines but try not to make them match up as you go, random pattern works best.
Other cracks:
For a more simple crack just scribe in by hand - practice on old CD cases.
This head light is made of very thick plastic so it's tough to work with - some are easier(Mirage) than others.