Prepare the testing environment

Save the following script as /opt/kde/runPlasma5.sh

#!/bin/bash
 
if [ -d /etc/X11/xinit/xinitrc.d ]; then
  for f in /etc/X11/xinit/xinitrc.d/*; do
    [ -x "$f" ] && . "$f"
  done
  unset f
fi

DISTRO=`source /etc/os-release 2>/dev/null; echo $ID_LIKE`
 
export KF5=/opt/kde/install
export QTDIR=/opt/qt5      # Change if you have your own qt build
 
export PATH=$KF5/bin:$QTDIR/bin:$PATH
 
export QT_PLUGIN_PATH=$KF5/lib/plugins:$KF5/lib64/plugins:$QTDIR/plugins:$QT_PLUGIN_PATH
#On some distros, this is $KF5/lib64/qml:$QTDIR/qml or something similar. After the installation, choose the directory/directories which contains the "qt/qml" folder.
export QML2_IMPORT_PATH=$KF5/lib/qml:$QTDIR/qml

#Fixes for Debian's "look at me, I'm so important" setups
if [ $DISTRO == "debian" ] 
then
    export QT_PLUGIN_PATH=$KF5/lib/`uname -m`-linux-gnu/plugins:$QTDIR/plugins
    export QML2_IMPORT_PATH=$KF5/lib/`uname -m`-linux-gnu/qml:$QTDIR/qml
fi

export QML_IMPORT_PATH=$QML2_IMPORT_PATH
 
export XDG_DATA_DIRS=$KF5/share:/usr/share
export XDG_CONFIG_DIRS=$KF5/etc/xdg:/etc/xdg
  
# Uncomment to change default home paths
#export XDG_DATA_HOME=$HOME/.kde5/local
#export XDG_CONFIG_HOME=$HOME/.kde5/config
#export XDG_CACHE_HOME=$HOME/.kde5/cache
 
exec startkde

For what works on Debian Sid alongside with packaged KF5 nicely see: Using (parts of) self-built KF 5, Plasma 5 and/or KDE Applications on Debian

Configure dbus

Because we installed Plasma 5 to a custom path, we need to make dbus aware of these new locations.

Create a file /etc/dbus-1/session-local.conf with the following contents:

<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
<servicedir>/opt/kde/install/share/dbus-1/services</servicedir>
</busconfig>

Create another file /etc/dbus-1/system-local.conf with the following contents:

<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
<servicedir>/opt/kde/install/share/dbus-1/system-services</servicedir>
<includedir>/opt/kde/install/etc/dbus-1/system.d</includedir>
</busconfig>

Because dbus will load all these new paths each time you login, you should rename these files (to eg system-local.conf.bk and session-local.conf.bk) before starting your default Plasma 4 or other distro sessions.

Running Plasma

Finally, there are several ways of running Plasma 5. Choose the one which suits you best.

Using a login manager

The recommended login manager is SDDM. But this method should work with any login manager.

sudo cp /opt/kde/runPlasma5.sh /usr/share/xsessions
sudo chmod +x /usr/share/xsessions/runPlasma5.sh
sudo cp /opt/kde/install/share/xsessions/plasma.desktop  /usr/share/xsessions/plasmaGit.desktop

Edit /usr/share/xsessions/plasmaGit.desktop and change the paths as:

Exec=/usr/share/xsessions/runPlasma5.sh
TryExec=/usr/share/xsessions/runPlasma5.sh
Name=Plasma Git

After this logout of your current session, select Plasma Git in the options and login. You should now have the last Plasma 5 running.

Using ~/.config/plasma-workspace/env

This is what I found works on Debian Sid nicely. Put the script which sets up the environment into ~/.config/plasma-workspace/env, chmod +x it and be done. Aside from the "Using a login manager" approach probably – I didn´t test it –, this is the only thing that worked on Debian. Additionally to that it does not need any changes to system wide configuration files. This directory replaced ~/.kde/env from KDE SC 4 and older times.

Specifically none of the put it into some user wide or system wide shell profile files worked. Also putting it at start of /usr/bin/startkde worked, as environment still got reset to the default.

From the command line

- Edit your xinitrc file :

mv ~/.xinitrc ~/.xinitrc.bk                # Backup any existing xinitrc file
ln -s /opt/kde/runPlasma5.sh ~/.xinitrc

Log out of your current session, login to a tty (eg tty2 by pressing Ctrl+Alt+F2) and run startx. This should read your ~/.xinitrc and log you into a Plasma 5 desktop.

- Instead of editing your xinitrc, you can simply override it by running startx /opt/kde/runPlasma5.sh.