Once you set up your wallet on ZERO, you can scan a QR code on its screen to sync the accounts with the LIQUID app. Your private keys and seed are never exposed, only the public addresses are shared with LIQUID. From then on, if the user wants to sign a transaction, you just open the app, fill in the transaction details, show a QR code to your ZERO, sign the transaction request on the device and show a signed QR code back to the LIQUID app to broadcast the transaction to the blockchain.