If you look at the replacement one, there is a spring loaded pin. Hold the new lock ass up to the steering coloumn and you will see where to drill the hole (3mm drill sounds about right). Drill the hole untill you hit the ign barrel, you will see from the replacement switch that the key has to be in a certain possition to push the pin in. Turn the key into this possition ,press in the pin through the hole and remove the key and barrel. Make sure the replacement barrel is in the same possition and fit it in.
Steve.