How to install SSL certificate on PLESK 12
1. Add certificate to a repository navigating to Plesk > Domains > example.com > SSL/TLS Certificates and select Add SSL/TLS Certificate
2. Then, enable SSL support at Plesk > Domains > example.com > Hosting Settings
3. At the same page, select a required certificate for a website at Plesk > Domains > example.com > Hosting Settings and click OK
Your website will now be secure using SSL certificate.