Download and enable like any other Drupal module.
External libraries: none.
Configuration: (D8/D9) some map providers require an API key or access token, which you can enter at …/admin/config/system/leaflet-more-maps. This is also the page to go to if you wish to assemble your own custom map layers.
Dependencies: as the name suggests, you do need to enable the Leaflet module. After doing that check the Reports >> Status Report page to verify that it confirms you have 20+ new maps on your system.