You can use ``trezorctl`` to sign a transaction without it automatically being broadcast to the Bitcoin network.
You will need the following pieces of info:
1) Transaction ID containing the Output we want to spend (aka ``prevhash`` or ``a5ea715a...d201e64e`` in example below).
2) Index number of the Output being spent from the above tx (aka ``previndex`` or ``0`` in example below).
3) BIP32 path to the Node which can spend the above UTXO (eg ``Bitcoin/0'/0/0`` for the first).
4) Destination address where you want to send funds (eg ``3M8XGFBKwkf7miBzpkU3x2DoWwAVrD1mhk`` below).
5) Amount to send in satoshis - ``91305`` in the example below (multiply BTC amount 0.00091305 by 100,000,000).
6) Expected fee (``0.00019695`` BTC in example below). Note: the miner receives all satoshis left unspent from a transaction. If you want to receive some change, you need to send it to an address you own (otherwise it will go to miner). Fee is not needed below, we just want it as a sanity check.
There are many ways to retrieve the info above: from a watch-only wallet in Bitcoin Core, https://coinb.in (`screenshot <sign_tx-coinb.in.png>`_) etc. The easiest way is using the Trezor online wallet: https://beta-wallet.trezor.io
After authenticating, open the "Send" tab, fill-out all details, then open the "Show transaction details" menu to see the info needed above (`screenshot <sign_tx-trezor.io.png>`_). Once you have the required details, you can then perform the transaction signing using ``trezorctl`` as shown in the example below:
Use the following form to broadcast it to the network:
https://btc-bitcore1.trezor.io/tx/send
The signed transaction text can then be inspected in Electrum (`screenshot <sign_tx-electrum2.png>`_), `coinb.in <https://coinb.in/?verify=01000000014ee601d273df3fe31f4bd79ad483d89973fe2d626faff31605a39ca95a71eaa5000000006a47304402206386a0ad0f0b196d375a0805eee2aebe4644032c2998aaf00e43ce68a293986702202ad25964844657e10130f81201b7d87eb8047cf0c09dfdcbbe68a1a732e80ded012103b375a0dd50c8dbc4a6156a55e31274ee0537191e1bc824a09278a220fafba2dbffffffff01a96401000000000017a914d53d47ccd1579b93c284e9caf3c81f3f417871698700000000#verify>`_ or another tool. If all info is correct, you can then broadcast the tx to the Bitcoin network via the URL provided by ``trezorctl`` or Electrum (Tools → Load transaction → From text. Here is a `screenshot <sign_tx-electrum1.png>`_). TIP: Electrum will only show the transaction fee if you previously imported the spending address (eg ``16ijWp48xn8hj6deD5ZHSJcgNjtYbpiki8`` from example tx above). Also, the final tx size (and therefore satoshis / byte) might be slightly different than the estimate shown on beta-wallet.trezor.io
The final broadcast and mined transaction can be seen here: https://blockchain.info/tx/270684c14be85efec9adafa50339fd120658381ed2300b9207d0a0df2a5f0bf9
