

Enable Administrator password on the menu screen, create a persistent storage (if it doesn’t already exist), download the Flatpak file from the website, and run
torify flatpak install /path/to/file
flatpak run io.github.softfever.OrcaSlicer
Using an AppImage is not a good idea because they have a tendency to give errors if proper software and version are not installed on Tails (on my Tails USB, this was because of GCC) unless you compile your own AppImage. Using Flatpak is better as it allows you to run software on your system even if the versions of GCC etc. are not up to date.
Please keep in mind that I have not confirmed whether this method is secure and would advise that you consider whether this is secure or not depending on your threat model.

That data might be easily accessible, but that was a choice Root made. I think that it is a safe assumption that Root knew most vigilantes keep their identity secret and, assuming a German background, had read Section 202 of the StGB and other relevant laws and court rulings. As such, Root most likely did this despite knowing their identity is at risk. It is likely they did this publicly specifically to inspire others, though I haven’t looked at all the details and there might be a different reason.
Nothing in this comment constitutes legal advice.