No idea how to get started with Xephyr. What am I doing wrong?

[...]

Hello.  I am a moderately experienced Linux user, running Mint 15.

I want to use Xephyr to make a multiseat setup with one card.  I followed a
couple tutorials online without understanding anything (just hoping it
would work), but it didn't.  So, I decided to try to get the most simple
thing possible with Xephyr to work and build it up.  But I can't even do
that!

I built 1.12 (from X11R7.7) using the instructions here:
http://www.amxl.com/c/single-graphics-card-multi-seat-ubuntu-linux

In my widows manager, I open a terminal and do:

$ Xephyr -ac -br -noreset -screen 800x600 :0
>

Fatal server error:
> Server is already active for display 0
>     If this server is no longer running, remove /tmp/.X0-lock
>     and start again.
>

I expected that, so I go to tty1 and

$ sudo service mdm stop
$ export DISPLAY=:0
$ Xephyr -ac -br -noreset -screen 800x600 :0
 Xephyr cannot open host display.  Is DISPLAY set?
$ echo $DISPLAY
 :0
$ DISPLAY=0 Xephyr -ac -br -noreset -screen 800x600 :0
 Xephyr cannot open host display.  Is DISPLAY set?

Then I did sudo su and tried all those again, still to no avail.  I've also
tried all this on another computer with Xephyr from the mint repo and
building 1.9 as described above.

Okay.  I have no idea.  How do I get a Xephyr hello world?

Thanks,
 Elliot
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