From the console
To start Plasma with xinit/startx, append export DESKTOP_SESSION=plasma and exec startplasma-x11 to your .xinitrc file. If you want to start Xorg at login, please see Start X at login.
To start a Plasma on Wayland session from a console, run XDG_SESSION_TYPE=wayland dbus-run-session startplasma-wayland.[1]