QUICK INFO:

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• General principle:

  • 1. In the hot (i.e. connected to the Internet) computer (better if it is a secure one with Tails as OS), set up the transaction in the "watch-only" wallet A ("...-A-WO") :

    • "Send" > paste address and set amount >"Pay..." > "Advanced" > set fee >...

  • 2. Then export the ".psbt" transaction to a file > save that file in a USB stick:

    • ... "Finalize" > "Export" > "Export to file" > select destination > "Save"...

  • 3. In the cold computer (Electrum wallet A with private keys: "...-A" wallet), import the ".psbt" transaction and sign it:

    • ... "Tools" > "Load transaction" > "From file" > select file > "Open" > "Sign" > paste/write password > "OK" > check that the status has changed from "Unsigned" to "Partially signed (1/2)"...

  • 4. Then export the partially signed transaction ("....psbt" file):

    • ... "Export" > "Export to file" > select location (e.g. USB stick) > "Save"...

  • 5. In the cold computer (Electrum wallet B with private keys: "...-B" wallet), import the partially signed bitcoin transaction ("....psbt" file):

    • ... "Tools" > "Load transaction" > "From file" > select file > "Open" > "Sign" > paste/write password > "OK" > check that the status has changed from "Partially signed (1/2)" to "Signed"...

  • 6. Then export the signed transaction ("....txn" file):

    • ... "Export" > "Export to file" > select location (e.g. USB stick) > "Save"...

  • 7. In the hot computer (any wallet as long as it is connected to your node, or to a trusted node), broadcast the signed transaction:

    • ... "Tools" > "Load transaction" > "From file" > select file (".txn") > "Open" > "Broadcast".