In order to install SSL certificates on your Apache server you need to generate a key pair and a CSR (certificate signing request) as the first step. The following points will guide you in the creation of CSR file.
Once you finish generating the CSR file, you need to paste the content of the CSR file on to the SSL certificate request page in order for the certificate authorities to start their verification process.
Command line prompts in a Linux box
Find where openssl is installed and navigated to that directory
$ whereis openssl
Mostly it will be at /usr/bin/. If it is in a different path, then navigate to that directory path
$ cd /usr/bin