ok, so:

1-edit garmindevice.xml from nuvi37xx and 430 to nuvi3790 and 390 version. only thats fields?
2-run webupdate
3-cut and paste GUPDATE.GCD, just download by webupdate, from .system to a SD card? in the root of sd card? or in some folder? which one?
4-unplug it from usb and turn on.

thats correct?