Some ideas:
NEMO has some resources on this topic:
http://www.firstnemo.org/resources.htm
Many teams have generously posted GREAT Team Handbooks in the white papers on this site. [I'm working on a paper on Great Handbooks for NEMO but not ready yet]. Look through the format and examples of what other teams have created and see if any of these seem to be a good fit for YOUR team.
And the nice thing to do is credit the teams/resources that helped you create your document.
Documentation is an incredibly important step in sustaining a team.