You basically open your save data in Raw JSON editing mode, copy the full base data part to clipboard, which can then be imported into Blender by the plugin. The plugin contains copied models (simplified with reduced meshes for performance) as objects and uses your base data to place them in Blender. You can add/remove/edit or whatever you like and once you are satisfied, the plugin can export this back to base data (JSON) to be used in the save again. You get a pretty decent view of what your base would look like.
Check out the Github for the plugin, it explains a lot better and even includes some video links: No Man's Sky Mods - #70 by DevilinPixy