Use the NetInstall, behind "More" on https://trisquel.info/download
Its installation will provide you a minimal (text-only) system (unless you ask for more during the installation process; I do not think an Xfce desktop environment is proposed) in which you can boot, log into, and execute:
$ sudo apt install xfce4Also, GNOME Shell is available for Trisquel 8 (many of us actually use it): https://trisquel.info/en/wiki/installing-gnome-shell
