On the back of the door-skin is a secondary skin which needs to be removed. This skin is only held on with an adhesive coated foam strip and after the drilling has taken place, it will be easy to remove. Click on the image below to see how it is removed....
You will see in the photo that there are 3 steps involved. The third and final step shown above, is to bend the two "tabs" back so that they allow the backplate to sit flush against the door skin..