Once the cassette has been fixed on the trainer Avanti, Justo, Justo 2, Direto XR, Direto XR-T, Suito, Suito-T (or if you have the trainer Direto XR and Suito with pre-assembled cassette) and you need to remove the freehub:
use the KIT KEY + ROTATING RIGHT NUT KIT to correctly disassemble it.
Kit is available at this link:
Kit tool + rotating right nut Justo, Direto XR, Direto XR-T, Suito, Suito-T
Here below the how to video to remove it: