G2, G3, G4, Flex - The Power and Volume Down buttons can be found on the back of the phone, underneath the camera lens. Optimus G, Volt - The Power button can be found on the right side of the phone, and the Volume Down button on the left.
While QuickMemo+ comes preloaded on most LG phones, your carrier may have removed it. If you have a custom operating system installed on your LG phone, you may not have QuickMemo+. Even though the Notification panel is open, the screenshot will be taken of whatever is underneath it.
Sharing a memo will save it automatically.