Create ratpoison.desktop file in /usr/share/xsessions/
[Desktop Entry]
Version=1.0
Encoding=UTF-8
Name=ratpoison
Name[en_US]=ratpoison
Comment=Simple and fast window manger
Exec=ratpoison
TryExec=ratpoison
Type=Xsession
[Window Manager]
SessionManaged=true
Make a symbolic link of /usr/bin/ratpoison file in /etc/alternatives/
sudo ln -s /usr/bin/ratpoison /etc/alternatives/x-window-manager
sudo ln -s /usr/bin/ratpoison /etc/alternatives/x-session-manager