you can name the xml file like you want. Common is something like "script-<addon_name>-main.xml". For your dialog I would use "script-<addon_name>-dialog.xml".
It should be placed in the path "<addon_dir>/resources/skins/Default/720p" to open it with:
Addon = xbmcaddon.Addon('<addon_id>')
__path__ = Addon.getAddonInfo('path')
gui = GUI('script-<addon_name>-main.xml', __path__, 'Default')
The first two lines are just to demonstrate how to get the path and could be done in a different way.
GUI has to be a python class which inherits from "xbmcgui.WindowXMLDialog".
You can have a look to my (not very good) demo script: https://github.com/dersphere/script.demo
But this is a full script - I think you want something to be returned from your dialog so its not a very good example.
Just have a look how other addon authors do something similar you need.