Migrating into a child domain.

Hi, does anyone have any ideas, tips, gotchas, etc… when moving from an existing forest into a child domain? Currently the college were I work has a 2003 AD and they want my division, the library, to move into this AD as a child domain. I’m currently running 2003 AD on my own, a AD forest. I know of ADMT but any ideas on what I should do first, second , etc… I’ve got a plan of sorts and I’m researching as I post this question. I want to minimize down time for my users, who are all connected to a print server and file server, they have home directories.
Any thoughts would be appreciated. Thanks.