Plug the programmer into the OBD-II port with the key in the off position. Follow the instructions on the programmer. It's vital that you don't unplug the programmer while you're tuning your Mustang. If you do, the computer could be left in an unusable state. A modern car without a functioning computer is a doorstop. Before taking your Mustang to the dealer for service return it to the stock tune with the programmer. If the dealer has to flash your computer, the programmer won't be able to tune your computer.