https://www.raspberrypi.org/forums/viewtopic.php?p=890408#p890408

This next part focuses on installing a GUI on top of Raspbian Lite. In order to have a GUI, we need these 4 things:
1. Display Server
2. Desktop Environment
3. Window Manager
4. Login Manager

1. Xorg Display Server
2. Raspberry Pi Desktop (RPD) or Lightweight X11 Desktop Environment (LXDE) or XFCE Desktop Environment (XFCE) or MATE Desktop Environment (MATE)
3. Openbox Window Manager (RPD/LXDE) or XFWM Window Manager (XFCE) or Marco Window Manager (MATE)
4. LightDM Login Manager

sudo apt-get install --no-install-recommends xserver-xorg xserver-xorg-legacy
#When not using login manager, install xinit:
sudo apt-get install --no-install-recommends xinit

Stripped down version of RPD (Raspberry Pi Desktop):
sudo apt-get install --no-install-recommends raspberrypi-ui-mods lxsession

optional:
pi-greeter : The Raspberry Pi LightDM login theme
rpd-icons : The Raspberry Pi Desktop icon theme
gtk2-engines-clearlookspix : GTK Theme Engine (used to render Raspberry Pi LightDM login/desktop theme properly)

Openbox WM is installed by default when using RPD.

with xinit, system starts in command line. When required, use startx to start GUI

Single application setup:

sudo apt-get install --no-install-recommends xserver-xorg
sudo apt-get install --no-install-recommends xinit
sudo apt-get install openbox lxterminal
#sudo apt-get install lightdm
echo "exec openbox-session" | tee ~/.xinitrc

sudo apt-get install linphone

sudo dpkg-reconfigure xserver-xorg-legacy

set anybody

sudo vi /etc/X11/Xwrapper.config

allowed_users=anybody
needs_root_rights=yes

sudo usermod -a -G audio,tty,video,users pi

sudo vi /etc/xdg/openbox/autostart

/usr/bin/linphone & #set the program to start

#disable right-mousebutton in openbox: 
sudo vi /etc/xdg/openbox/rc.xml

#comment out these lines using <!-- and -->

<!-- 
#    <mousebind button="Right" action="Press">
#      <action name="Focus"/>
#      <action name="Raise"/>
#      <action name="ShowMenu"><menu>client-menu</menu></action>
#    </mousebind>
-->

sudo vi /etc/systemd/system/kiosk.service

[Unit]
Description=Kiosk

[Service]
Type=oneshot
ExecStart=/usr/bin/sudo -u user /usr/bin/startx --

[Install]
WantedBy=multi-user.target

sudo systemctl enable kiosk
sudo systemctl start kiosk