The first step is to create a Certificate Signing Request (CSR) on the Synology Diskstation.
Then I'm uploading the CSR on my Virtual Machine where the Root-CA is "running" on. As Root-CA I'm using a GNU/Debian Linux with Xca (X - Certificate and Key management) installed.
After the signed certifcate is created (and transferred from the virtual machine) I am just importing it on the Synology Diskstation.
After the restart of Synologys internal web server I am verifying if the certificate is working as expected.