It's pretty much the same process as described earlier in this thread, except instead of using a second computer as your airgapped device, he is using a phone.

This is certainly better than just sweeping your private key in to a hot wallet, but it is not as good as using an airgapped computer for a couple of reasons. A phone in flight mode is not truly airgapped, and studies have shown it still pings for cell towers, it can still send location data, it can still leak via WiFi and Bluetooth, and so on. A phone is not truly airgapped unless you open it up and start ripping out the antennas and WiFi modules. You also are not starting on a clean device, and your phone could have any type of malware on it already. Even if you factory reset your phone, you aren't starting from scratch, but rather starting from whatever bloatware and spyware your carrier/manufacturer has pre-installed, and with their (usually) closed source OS.

An airgapped phone is an OK middle ground, but it is not as good as using a fully airgapped computer with a clean install of an open source OS.