Summarize:

This is the yellow box with a white ghost icon on your Home screen, or in a folder on your home screen. If you have not already installed Snapchat and created your account, please do so before continuing. Snapchat always opens up to the camera screen, and swiping right will take you to your Chat page, where you can view the snaps you received. This will open the snap, and you will have 1 to 10 seconds to view it. You can only view every snap once, and you get one replay every day. You won't be able to view or screenshot a snap that you've already opened and closed before, unless you replay the snap immediately after it expires. Hold down the Sleep/Wake and Home buttons at the same time and release them. You will hear a camera shutter sound and see your screen flash, indicating you just took a screenshot. Your screenshot of the snap will save to your Camera Roll.
Open Snapchat. Swipe right. Tap on the snap you want to save. Take a screenshot before the snap expires.