For now, you can only access the feature through the Gmail website, not the mobile apps. When you're ready to send a self-destructing message, head to the site, log in, and start creating an email as usual—click Compose on the top left of the page and then write your message. Next, click the Confidential Mode button, which looks like a padlock icon, at the bottom of the Compose window. After that, choose when your email should expire and erase itself: A day, week, month, three months, or five years from now. If you're sending to a non-Gmail user, this is the point when you decide whether to send the passcode via email or text message. For the latter option, you'll need to enter a cell phone number. Finally, click Send.