Creating a new SPObject
Copy sp-skeleton.cpp and sp-skeleton.h to your files, and rename everything from /skeleton/i to your name.
Add your object name to the list in sp-object-repr.cpp's repr_name_entries, and add the new .h file to the include list at the top. This will let your new SPObject have it's functions get called when a Repr appears in the tree.
Note: it must be added to repr_name_entries NOT sodipodi_name_entries.