There are two fundamental approaches.
First is to collapse and consolodate your existing Windows NT domains into
one. This has the advantage of making the migration to Active Directory
much simpler. It also means that you focus time and energy cleaning up your
current Windows NT environment prior to installing Active Directory. Once
you have the Windows NT environment at the place you are comfortable with
it, you can perform an upgrade-in-place. Essentially, that means upgrading
the single domain's PDC and making it the first DC in a new Active
Directory. Once done, you can install or upgrade additional DCs. This is
an approach I have taken with a number of networks and it has been very
successful.
Another approach is to upgrade the individual NT domains. When you do this,
you have the ability to build them into a forest structure as you see fit.
Once done, the objects in the domain can be migrated easily into another
single Active Directory domain and the old domains decommissioned.
There are any number of variations on these two - but the one thing I
encourage you to do is spend plenty of time planning the details.
-ds
