There is no secret to burn CDs from cassette tapes. If you want to transfer their content to the CDs, a recording procedure needs to be done before. You need to connect the device which plays the cassettes to the PC through LINE IN and use a recording application to capture the audio stream. Once the files have been saved, you can use a burning application to transfer the files to CDs or DVDs. For recording, I recommend you to use Audacity as it's a very simple file recorder.