1. Reset the phone you want to use for cold storage. Settings > System > Reset Options > Erase all data (factory reset)
2. Install COINiD Vault. #Play store URL#
3. Remove SIM card if you have not already. Then forget all Wi-Fi networks. Settings > System > Reset options > Reset Wi-Fi, mobile & Bluetooth
4. Turn on Airplane Mode, activate Bluetooth and make sure Wi-Fi is disabled.