you can do this using Electrum or using https://www.bitaddress.org/

 - Download an open-source (Linux) system using the official URL.
 - Download Electrum using the official URL & verify the signature.
 - Boot your OS from an offline PC (It is better to remove internet Hardware parts) and install Electrum wallet (that contain the private key).
 - Create a watch-only wallet.
 - use online PC to access to that wallet (watch only).
 - Create a new transaction ----> click Preview ----> check everything is correct ----> click save or generating a QR code.
 - open your offline OS -----> open Electrum ----> Load Transaction or use a QR code  ----> check it is correct -----> click Sign ----> enter your password.
 - back to your online PC ---> Load Transaction ----> Broadcast it.