I've yet to find a single comprehensive document for creating Custom Data Objects (CDO), but here are the ones I've found most helpful:
Best Practice – Custom Data Object Records (Data Cards) - Definition and Recommendations for Usage:
Best Practice – Custom Data Object Records (Data Cards) - Definition and Recommendations for Usage
Info for setting up a new CDO via the form processing step: http://topliners.eloqua.com/servlet/JiveServlet/previewBody/1845-102-1-2247/E9%20Forms%20Processing%20Steps.pdf
Setting up CDO in the context of someone's ADC email setup:
http://topliners.eloqua.com/community/do_it/blog/2013/08/30/how-to-create-an-adc-email
I hope that helps!
