The nice thing is that there is a plugin which was made to serve as an example for this: the uiSeisIOSimple plugin. SEG-Y import/export is rather complex due to the complexity and incompleteness of SEG-Y which makes it not very good as an example.

When the plugin was made, documentation was added to show that you can package your own doc with a plugin. This all shows pretty well what is going on. The plugin supports 3D as well as 2D data. The plugin is also very useful by itself: it allows I/O from simple ascii or binary files.

When this plugin still looks complex, it is only like that because the handling of the 'simple file' data requires some work. The OpendTect stuff is rather straightforward: you need a SeisTrcReader for export and a SeisTrcWriter for import.

Concluding, look at plugins/uiSeisIOSimple, avoid the SEG-Y stuff in uiSeis like uiseissegyimpexp.cc.