You only need to build layers in Inkscape if you’re using multiple floors. Check out this thread. Otherwise all you need to do is create objects that correspond to the entities you’ve listed in your floorplan.yaml e.g. a light bulb icon with object id ‘light.bedroom’, or in my case black rectangles with the object IDs of the lights they represent.
If you’ve installed floorplan correctly you should already have custom_ui/floorplan/floorplan.css and you just need to modify it. If you need more specific help give us a look at your config and floor plan.