1. Login in Plesk with admin role
2. Go to Tools & Settings => Security => SSL Certificates . A list of SSL certificates that you have in your repository will be displayed.
3. Click Add
4. Specify the certificate properties:
   - certificate name (a mnemonic to identify your certificate in the repository)
   - Encryption level (choose 2048 bit)
   - Specify your location and organization name (max 64 characters)
   - specify the hostname of your Plesk installation
   - specify your email address
5. double check all the informations you provided, because depending on the kind of your SSL vcerificate (DV, OV or EV) these information will be included or not in your certificate
6. click Request
   Plesk will generate your private key and CSR (Certificate Signing Request) and store them in the repository
7. click the name of the certificate you need; a page with certificate's properties will open.
8. Locate the CSR section, and copy to the clipboard all the text (including "-----BEGIN CERTIFICATE REQUEST-----"  at the beginning and "-----END CERTIFICATE REQUEST-----" at the end
9. order your SSL certificate on DomainRegister.international
   (What kind of SSLcertificate? We suggest you a cheap DV SSL Certificate, or a most professional and wide recognized OV SSL Certificate. Usually a much more expensive EV SSL Certificate is not worthwhile for such kind of application.)
10. When you receive your new SSL certificate, return to Plesk's SSL Certificates Repository, select your certificate and click Upload Certificate
11. Copy and paste in plain text the certificate (*.crt) and the CA certificate (*-ca.crt) which were provided you together, and then click Upload certicate
12. Once you have uploaded your new cerificate to the repository, select the corresponding checkbox and the click Secure Plesk

It's all: now your Plesk is secured by the new certificate, and the green closed lock will appear in the navigation bar of the browser.