The basic syntax you need to add to your Apache VirtualHost looks like:

SSLEngine on
SSLCertificateFile /path/to/example.com.crt
SSLCertificateKeyFile /path/to/example.com.key
SSLCACertificateFile /path/to//intermediate.crt

For more details, check out the tutorial "How To Install an SSL Certificate from a Commercial Certificate Authority" It should walk you through all the steps.

Let us know how it goes, and if you're still hitting any issues.