Hold the lock button and press the home button, then release the lock button. You should have the screen flash white if it works; photos go to the camera roll.
P.S. It's 3G. G3 was a processor Apple used... about eight years ago.
It's a nice feature... it was good that Apple provided it up to everyone. Enjoy!
P.S. To be really trick, I use it with the Photobucket app to send screenshots directly to my PB. The only problem with that app is it's kind of a nuisance to move to a subfolder.