After download - suggest the full map not the separate zips, takes no longer overall - and then unzip directly to the SD card not to a PC directory and copy across. You need a 32gb or larger formatted SD card
Once in the car the update takes about 40 minutes, do as a single update - keep the car turned on and do not interupt the update. Go for a long drive if you think your battery will not last, most will easily.
This way there are the fewist steps to get wrong and cause a c0ck up.
Worked for me and I was also unsure of the process