I think the primary confusion in the document is that the original private key that was used to generate the Certificate Signing Request (CSR) is needed. Often, the steps used to generate the original CSR are forgotten or some application generated it and did not offer the original private key.
Here's the steps reworked a bit:
You need access to the openssl command which is open source software and comes with most versions of linux; you can likely obtain versions for other OS needs on the internet. You may be able to use another certificate generating software, but our instructions focus on openssl.
You also need the original private key that was used to generate the Certificate Signing Request (CSR). Once generated, the CSR is sent to the Certificate Authority (CA) who uses the CSR with their own private key to generate the finalized certificate which contains information from both private keys. The original private key (yours) will often have a passphrase that you will need in order to generate the necessary unlocked key.
1. Delete the passphrase from the key used to generate the certificate signing request.
You can use the following openssl command:
# openssl rsa -in key.pem -out key-nopass.pem
where key.pem is the input filename of the key, and key-nopass.pem is the output filename for the key without the passphrase. You will need the original private key passphrase for this step.
2. Create a new text file.
3. Copy the certificate you obtained from the Certificate Authority into the new file (the main cert).
4. Below the certificate, copy the information from step 1 into the new file so it has the contents of both the main cert as well as the key-nopass.pem.
5. Save the new file locally.
6. Import the file into the Control Center.
If you do not have the original private key passphrase, you will not be able to complete the steps above (unless it has no passphrase to begin with). The original private key is something that should remain secure, you may want to delete the version without the passphrase when the steps are completed and store the original in a safe place.
Hopefully that helps you complete the steps.