Go to domains and SSL:
Manage SSL:
Select the certificate and then choose the domain and select use with my on server:
Download private key:
Download the SSL certificate and the Intermediate Certificate:
Head over to plesk and go to the domain and go to SSL/TLS Certificates:
Go to advanced settings (this could be in 2 different places):
Selects add SSL/TLS Certificate:
Fill out the relevant details (Name, Domain…) and upload the files (Private key goes in Private key, Certificate goes in Certificate, Intermediate certificate goes in CA Certificate) and upload it:
Go to hosting settings:
Select the certificate you made (tick Permanent SEO-safe 301 redirect from HTTP to HTTPS) and go to the bottom and click continue:
And that is all.