In order to install an SSL cert after purchasing follow the steps below
This article assumes you have already generated a CSR and purchased as certificate. If you have not done this, please see the related articles
1) Log into your plesk panel
2) Click the relevant domain
3) Click 'SSL Certificates' (or 'Secure your sites' if you are using an older version of the plesk panel)
4) Click on the certificate that you wish to work with
5) Paste in the certificate text into the box provided
6) If your vendor has provided you with a CA Certificate, paste that into its corresponding text box. Not all providers will supply this.
7) Once pasted in, press 'send text'
8) Click Hosting Settings next to your domain name in Plesk, make sure that SSL is enabled and select your new certificate from the drop-down box.
This should complete installation of your cert
Some providers rather than a text based cert will provide you with a .crt file. In this case, just use the browse button below Upload Certificate files and press send file rather than send text above