For SSL/TLS negotiation to take place, the system administrator must prepare the minimum of 2 files: Private Key and Certificate. When requesting from a Certificate Authority such as DigiCert Trust Services, an additional file must be created. This file is called Certificate Signing Request, generated from the Private Key. The process for generating the files are dependent on the software that will be using the files for encryption. For a list of the server softwares DigiCert has, look at: DigiCert CSR Generation.
Note that although certificates requested from Certificate Authorities such as DigiCert are inherently trusted by most clients, additional certificates called Intermediate Certificate Authority Certificates and Certificate Authority Root Certificates may need to be installed on the server. This is again server software dependent. There is usually no need to install the Intermediate and Root CA files on the client applications or browsers.
Once the files are ready and correctly installed, just start the SSL/TLS negotiation by using the secured protocol. On browser applications it is usually https://www.digicert.com. Remember to use your secured website address. Above is just a sample address.