Several people have asked me about some advice on creating custom classes inside ObjectARX.
I have made a detailed tutorial and presented it as a Class last year at Autodesk University.
There is an online video recording, PPT and PDF of this class at AU2008 website:
For your convinience, I have also posted a direct link to the PDF:
This is the link for related source files:
This is the link for the Step6 converted to VS2010 SP1, using Toolset V90 and ObjectARX 2012. You will also need VS2008 SP1 installed to be able to compile it.