You need to build and install lightdm and lightdm-kde in the same prefix. We are going to assume both are installed in $PREFIX

You can either do:

cd $PREFIX/share/xgreeters/
ln -s lightdm-kde-greeter.desktop default.desktop

or:

• edit $PREFIX/etc/lightdm.conf
• find the group "[SeatDefaults]"
• set the "greeter-session" key to "lightdm-kde-greeter"

But your changes will get overwritten next time you run make install in lightdm directory.

Lightdm needs to know which sessions are available:

cd $PREFIX/share
ln -s /usr/share/xsessions

Start lightdm like this:

$PREFIX/sbin/lightdm --test-mode --debug

Output of the greeter is sent to $HOME/.cache/lightdm/log/x-$N-greeter.log, where $N starts at 1 and increase each time you close the window. $N does not increase if you stop lightdm with Ctrl+C, so using Ctrl+C is the recommended way to stop it.

Note: the test X server is using a QWERTY layout. Keep this in mind if you are using a different layout when you type your password.