Mostly got this working.
Install the LibreSign App: https://cloud.example.com/settings/apps Search for "libresign" with the top eyeglass icon search.
Go to its settings and download the dependancies:
https://cloud.example.com/settings/admin/libresign
Copy-paste and run the following commands from the settings in the Cloudron Terminal for that app:
https://cloud.example.com/settings/admin/libresign
occ libresign:install --java
occ libresign:install --jsignpdf
occ libresign:install --cfssl
Ignore the last command it says in settings: occ libresign:configure:cfssl --help, unless you know more about that. Instead, use the button to Generate a Root Certificate, and enter the appropriate values to you in that. For personal Certs, you don't need an Organisation.
Everything seems to be working except signature images. If anyone knows why or how to fix that, be greatly appreciated.