Mods folder, default, GFX, Models, dump textures (and models) there
Mods folder, default, GFX, Thumbnails, dump thumbnails there
Mods folder, default, Data, new .txt, paste new part definition, save as..., NAME.Xml(select all types so it save right)(Or copy GC2Types.XML, open in note pad and dtop the stuff in it then move the copy to the mods data folder, and remove the "- Copy" from it.)
launch game, shipyard, new/upgrade, enjoy!
Please correct me if I am incorrect.
EDIT:
Thse might help...
http://www.youtube.com/watch?v=pii66hLF-qQ&list=PLrZlBx189IbV7HwBypYvwuE8NDveDpAK5