As loldier & Magic Banana said just click on the Trisquel icon as you start a session.
Trisquel 7 mini has by default Trisquel mini + LXDE + Openbox as desktops environments and is perfectly fine with adding Xfce.
If you wish to have only one specific DE of your choice (those not proposed by Trisquel) , you can start a text mode install, install the base system, do your partitioning & then when comes to choose the DE you opt for Tasksel : At 'Select Software' choose Trisquel Console Enviroment , from there install what you need, as shown here
https://trisquel.info/fr/wiki/construisez-votre-propre-version-de-trisquel
