you don't... at least not on your offline wallet... You use your online (watch-only) wallet, that has been tracking all unspent outputs funding your watch-only address, to create an UNSIGNED transaction.
You then transfer the UNSIGNED transaction from your online (watch only) machine to your offline machine. Since the wallet on the offline machine can derive the private key(s) that correspond to the addresses loaded into your online watch-only wallet, it should be able to sign the unsigned transaction.
You then transfer the signed transaction back to the online machine for broadcasting.