Firstly, thanks to PowerShell.Org which helped to complete my scripts successfully; as a final step I want to add the digital signatures to the attached files , could someone please help me on this.
You will need a code-signing certificate for that. In order to be of any use for public sharing, that certificate will need to be issued by a trusted authority, which may cost you a bit of money, depending on your needs. (There are, however, some authorities that will issue free certificates for open-source projects, such as https://www.globalsign.com/en/ssl/ssl-open-source/ . I don't know if they'll issue a code-signing cert this way, but it's worth asking.)
Once you have a certificate, you just use the Set-AuthenticodeSignature cmdlet. As an example, here's the code I use to sign my scripts: