Well this is a most common question that most of the defence enthusiast post. So I am just creating this thread so that those who will in future post similar question can be merged here. As they say, you don't approach them, rather they approach you and absorb you in the department. But Officially, as a general placement , one has to be a government servant, be it both Class I(through Civil service) or Class II. They will see your background and place you in the wing. They also used to recruit graduates, directly from the Uni. Other deputations are from military and police. In a nutshell, to be honest, no one knows the criteria of recruitment perfectly. It is highly confidential and secretive. Other members may post whatever in public domain is.