Download once. Use anywhere.
Download maps from geofabrik:
http://download.geofabrik.de
Maps have the extension [.osm.bz2]
Extract your map into the folder:
sdcard/Android/data/com.khawarezm.offlinemaps/map.osm
In Android settings, grant OfflineMaps the permissions to access location and storage
Restart OfflineMaps so that it starts reading the [.osm] file; this may take some time depending on the size of the map.