Hold the power button like you would to do a reset and just press home once and it should flash white to show you've taken a screenshot.
To access them, connect the iPod to your computer and just go to 'My Computer' and go into the random named folder on your ipod with the screenshots in.
Level 99 Assassin
Bored of grinding on 78's? Get faster XP here!
Leveled from 98-99 in 19 mins 04 seconds
Currently waiting for Z2