1.install the fvwm,as root:
yum install fvwm
2.make a config file
vi ~/.Xclients
#begin of file
#maybe "exec/usr/bin/fvwm" could work also
exec /usr/bin/fvwm2
#end of file
3.change runlevel,as root:
rm /etc/systemd/system/default.target
ls -s /lib/systemd/system/runlevel3.target /etc/systemd/system/default.target
4.remove gdm,GNOME.as root:
yum remove gdm
yum groupremove "GNOME Desktop Environment"
5.start x windows
log in,then "startx"