you need a blank memory stick with Fat32 formatting, dont know if thats changed but start there.
Then your download will include a small FSC file which may be compressed, so decompress and extract.
When transferring and extracting the slightly compressed map files to a USB drive, this FSC file must be part of the package. From what I recall, map updates typically consist of several separate files. Again I havent done this in a very long time.
Remember the FSC code is VIN specific, if the FSC youre transferring is not valid for your headunit, you will brick NAV and corrupt your HUs directory. Youll then need to go to dealer to reflash the HU.
Tread with caution.