$ Xephyr -ac -screen 800x600 -reset :1 &
-ac : Disables access control restrictions and allows to forward the session -screen : sets the resolution of the new X session. Set it according to your need. -:1 : This specifies that the new X session is display number “1“. Your current Display is most probably “0“ -reset: Resets the X server when the last X client is closed.
The Xephyr should start and you will get a blank new window. The $DISPLAY environment variable tells the X client to which X server it should send its output to. Thus, $DISPLAY controls in which X server an X application pops up. As you must have figured it out, by default, $DISPLAY is set to “:0“.
Now, set the $DISPLAY to “:1” so that you can send output to the Xephyr window.
Then shoot up any X application and it will get displayed in the new X Server.
$ xterm & $ chromium &