Here is the procedure to convert an unused mobile into cold storage and access the funds from other device:
- 1. Install Electrum on both devices, and create a standard wallet on the "cold storage" device, that will be your main wallet that contains all the keys and shouldn't be connected to the internet even once (a newly formatted device/new device is better).
Make sure that the seed was saved in a physical backup like a piece of paper. - 2. Click the wallet's name on top of the screen and click "Master Public key" twice to open the QR code.
- 3. On the online phone, create a new wallet using the option "Standard Wallet->Use a master key", then click the camera icon to scan the cold-storage wallet's QR code.
This will create a watch-only wallet version of your cold-storage wallet. - 4. Confirm if the address in the receive tab is the same.
Now to use those wallets:
- You just have to create a transaction using the online watch-only wallet using the send tab.
- Fill out the recipient, amount, etc. then, click pay (select if you want to opt-in RBF, yes) and click the QR code icon below.
- In the cold-storage wallet, go to send tab and click the camera icon on the right side and scan the other device's QR code.
- The transaction will be imported to the cold-storage wallet, now click option->sign (enter your pin) and it will be marked as "signed" above.
- Click the QR code icon, then scan this using your online watch-only wallet and the signed transaction will be imported and now you can use options->broadcast button to send it to the network.
If you're not familiar with Electrum's defaut bitcoin denomination, you can change it from mBTC to BTC in the settings->denomination.