Does anyone have a clear, step-by-step explanation of the process by which maps are uploaded and updated on a typical SatNav device, including SD card?

I've used a Garmin NuVi 225W for a couple of years now and installed the initial maps by following the instructions blindly on purchase. I also purchased a lifetime sub to map data for Europe. I've been going through the updating process again recently, to be told that my device now hasn't enough memory to load all Europe, so - as a holding action - I installed North-West Europe.

I happened to have a spare 16Gb SD card and though that, if I put that into the Garmin, that would create the memory space needed to upload the whole file. However, never having used an SD card with a SatNav before, I'm unsure as to how the card is recognised by the device and by the installation system. Do I have to format the card first, install maps on it and then insert into the device? Or can I format and upload maps on to the card while it's in the device?

Technology doesn't usually phase me, but this one appears a bit of a maze!