To commence your confidential communication, tap the App Store icon from inside iMessage (it's the one farthest to the right, beside the compose window), hit the four dots in the lower left corner, select "Store," search for Confide, and download it. When you want to send a self-destructing text or photo, tap the App Store icon, swipe until the Confide composition window pops up, and type your missive.
Once you've hit send, the contents of your message will appear to the recipient as a series of orange censorship bars, and can only be revealed by tracing a finger across them. Once it's been read, the message disappears. Similarly, a photo you send through Confide is blurred out, and then promptly disappears once it's revealed. To protect against any funny business, every message is encrypted end to end, and if a recipient attempts to take a screenshot, it'll alert you and kick them out of the message (although, since the censorship bar ensures only a portion of the message is revealed at any one time, getting a damning screenshot in the first place would be tough).