You create a folder called Maps and inside it you can create two level maps: 1 you can name it Showcase or Demo if you have a scene and 1 you can name it Overview and you will place all assets which are part of the package over a platform. If you are only distributing materials/textures you will only need an Overview map inside your Maps folder.
I would suggest for you do download from the Marketplace the Automotive Materials (free Epic’s content) and see how it is done the Overview map.