Refer to the documentation.
1. Install kde windows manager.
Using pkg just do
pkg install kde5
proc /proc procfs rw 0 0
1.2. Enable hald (Hardware Abstraction Layer Daemon, provides services to the desktop) and dbus (Desktop Bus, an interprocess communications mechanism)
Enable them in vi /etc/rc.conf
dbus_enable="YES"
hald_enable="YES"
echo "exec ck-launch-session startplasma-x11" >> .xinitrc
Then in command line startx.
This is because KDE no longer develops its KDM (KDE Desktop Manager)
pkg install -y sddm
Then enable it in vi /etc/rc.conf with
sddm_enable="YES"
Tip
Shortcut to return to console CTRL+ALT+F1
Delete plasma file and restart.
rm -rf ~/.config/plasma-org.kde.plasma.desktop-appletsrc
Or you may move the file as a backup and restart.
mv ~/.config/plasma-org.kde.plasma.desktop-appletsrc ~/.config/plasma-org.kde.plasma.desktop-appletsrc-old