I want to contribute a patch to this feature request Consider adding Dvorak in the default keyboard layout option list in the greeter in Tails OS so I am thinking how to do it in Debian 8.7 first. I do not understand precisely how you can make such a change in the existing USB Live media.  I do know how to make a persistent live system as in the thread How do I make a persistent live USB of Ubuntu 16.04? but how to make a change in the existing live installation media is unknown to me because There is an installation code in apt-get which creates the installation on USB. It is using some codes. How can you study those codes which is about the keyboard layout in the greeter?...OS current: Debian 8.7OS target: Tails OS    
How to add Dvorak in Debian greeter of installation media CD/USB?
debian;keyboard layout;tails os;dvorak