Create the following directory structure under the App_Plugins folder:
App_Plugins/<Plug In Name>/Backoffice/Icons
If you don't have the App_Plugins folder, you may need to create one.
Umbraco's doc type icons are a collection of SVG files. In the same directory you've created, you may put the list of SVG files which are to be your custom doctype icons.
The directory and the files should look like the following:
Umbraco automatically detect these SVG files and list them as doc type icons with the file name used as the icon name.