First, you'll need to update your Gmail app to the latest available version.
If they want to turn confidential mode on they should press the button in the top right-hand corner that is shaped like a stack of three bullet points. You have the ability to set an expiration date as well as a passcode for the email, if you so choose.
With the new add-on installed, users can add a file to an email message by directly linking to a folder or a file stored in Dropbox. Once you click it, you'll see the various settings you can enable. However, one feature which was talked about a lot was the Confidential Mode.
However, it's worth noting that the new confidential mode doesn't stop anyone from just taking a screenshot of the confidential material, so keep that in mind. You can also bypass the passcode feature. I sent an email to myself with confidential mode, the attachment was a photo.