Apolgies for my slow response. I've been snowed under. I'm still not done, but the pressure has been relieved enough that I can come up for some air.

You note that you want a Rapid SSL certificate, although it's perhaps mentioning that it's possible to get free "Let's Encrypt" SSL certs via our integration, built into Confconsole (should "just work" in v16.x & v17.x; some tweaking is required in v15.x - please ask if you need more asistance following that path).

If you're sure that you want to use Rapid SSL, then you'll need to ensure that the certs are in the correct "PEM" format. I suspect Rapid SSL provides them in that format, but just in case, a few years ago I wrote a blog post about converting different certificate types to PEM format. The blog post also notes how our default certificates are constructed if you want to replace the defaults.

If you just want to add your cert to Apache for your website (you didn't mention, but I'll assume Apache is your web server - it is in ~70% of our appliances), you just need to put the certs somewhere (I'd suggest in /etc/ssl) and then in your Apache site file (have a look in /etc/apache2/sites-enabled to be sure) and add these lines within the HTTPS virtual host section:

SSLCertificateFile /etc/ssl/your_cert.crt
SSLCertificateKeyFile /etc/ssl/your_cert.key
SSLCertificateChainFile /etc/ssl/your_cert.ca-bundle

Obviously adjust the paths to match your files.

Hopefully that works. If you have any troubles or have more questions, pelase ask. Please be sure to include a bit more info about your server though. The easiest way is to share the output of 'turnkey-version'.