Sorry for just joining now. Documentation with images will follow. For now, here is version 1:
Create an account.
Go to settings.
Click on "Cloudron Instances".
Give your instance a name (just for labeling), add your IPv4 and IPv6 addresses, and save.
Click on "Access Tokens", add a token, give it a name and an expiration date, and copy the token.
Go to your Cloudron instance -> System -> Docker (https://my.example.org/#/docker) and add a new registry.
Fill it out: "Docker registry provider" = Other, "Server address" = apps.safeserver.de, "Username" = the email address of your account on apps.safeserver.de, "Password/Token" = the access token you created on apps.safeserver.de, and save.
Go to https://apps.safeserver.de/apps, click "View" on your desired app, and then "Install app".
Copy the "CloudronVersions.json URL".
Go to your instance, add a new community app (https://my.example.org/#/appstore), and paste the URL.
That's it. Simple, right!?