For starters you'll need to be an Administrator of the network -- this is required to duplicate the domain information over from the PDC. Basically setting a up BDC is quite straight forward depending on your reasons, if you are just doing it to distribute the authentication load and provide some redundency then it is just a matter of installing a new copy of NT4 Server and selecting "Backup Domain Controller" when selecting the server role. You'll then be prompted for some information, and need to supply Administrator credentials and the server will do the rest.
There are other reasons you could require and use a BDC, but they all start with those steps.
Regards,
Simon Clausen
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