I have smashed up two helmets so far...
Step one: Have an accident.
Step two: Go here.
Both mine were covered in blood but your method may vary. Make sure you specify (if it is important to you) the replacement COLOUR that you want your helmet to be in.
As an example: I came off using a red and black Giro. I sent it away, with cheque, and got back a blue one. (That was OK because I have a blue bike - no big deal).
The next time I came off (?I couldn't care less about the colour of the helmet) but "Bam" it went.
Sorry, there is a thread here somewhere and it shows the effectiveness of the helmet - it's probably been deleted by a non-helmet wearing Mod!