Enter your key and the target machine's code (Settings → License, looks like 8F3A-22C1-90DE-4471).
You'll get an activation file — USB it over, import, done. The machine never needs to see the internet.
On the target machine, copy the machine code from Settings → License.
On any device with internet (this one), exchange key + machine code for activation.json.
USB stick it back → Settings → Import activation file… → activated, offline forever.