How to use custom boxes plugins?

I'm assuming that you're asking to to implement a custom boxes plugin? Boxes leverages the ctools plugin infrastructure, so you'll need to implement hook_ctools_plugin_api() to tell ctools that you're going to provide a boxes plugin, and then hook_boxes_plugins() to actually register your plugin. The plugin itself is implemented as a class, and you should probably extend the boxes_box class.

It's this helpful?

I think the question is how do you then create a box of that type, once you've implemented it? You have to clear the menu router cache and then there's a tab under the blocks admin page to 'Add your box type'.