Dear Sir,
It is very easy. The procedure as prescribed Govt. of New Delhi is as follows. You may read and appreciate. The relevant link is provided here under for your ready reference.
Marriage Certificate and Registration
A marriage which has already been solemnised can be registered either under the Hindu Marriage Act, 1955 or under the Special Marriage Act, 1954. The Hindu Marriage Act is applicable in cases where both husband and wife are Hindus, Buddhists, Jains or Sikhs or where they have converted into any of these religions. Where either of the husband or wife or both are not Hindus, Buddhists, Jains or Sikhs the marriage is registered under the Special Marriage Act, 1954.
Further, marriage can be solemnised between any two persons under the provisions of the Special Marriage Act, 1954.
http://delhi.gov.in/wps/wcm/connect/DoIT/delhi+govt/community/marriage+certificate+and+registration