Click the Save button on the top level menu of the map view, then click Save As to save a new map or a new copy of an existing map: you will be prompted to give your new map a title and tags (required, but you can choose any you would like), and to add a summary, if desired. If you are working with a pre-existing map, just click Save to keep saving it to the same place – make sure to do this regularly while you are working so your changes are saved.