these instructions describe how to create a basic ltsp system where all the
applications are running locally with an anonymous login, using gdm, for i386
hardware.  non-anonymous login with ldap or some other authentiation mechanism
with writeable home directories is definitely possible, but not described here.
and of course, you could probably use kdm insead of gdm, with a few changes.

install the ltsp-server package:

apt-get install ltsp-server


build the client environment:

ltsp-build-client


install a desktop environment:

chroot /opt/ltsp/i386 tasksel install desktop
(alternately: chroot /opt/ltsp/i386 apt-get install gdm <and other packages>)


add an anonymous user for autologin:

chroot /opt/ltsp/i386 adduser --disabled-password --gecos ,,, autologin

(note: you may want to ensure a specific --uid and --gid, too)


make /home and /var/lib/gdm writeable:

echo 'copy_dirs="$copy_dirs /home"' >> /opt/ltsp/i386/etc/default/ltsp-client-setup
echo 'rw_dirs="$rw_dirs /var/lib/gdm"' >> /opt/ltsp/i386/etc/default/ltsp-client-setup


to /opt/ltsp/i386/etc/gdm/gdm.conf, add the following lines in the daemon
section, like so:

[daemon]
AutomaticLoginEnable=true
AutomaticLogin=autologin
TimedLoginEnable=true
TimedLogin=autologin
TimedLoginDelay=30