I would include:
Info/description of what a stage manager does/is.
What to do during auditions (even if they are not there)
Stage Managers and rehearsals
Basic forms used, etc.
And anything your organization expects of its stage managers.
I would make the guide a two parter.
One that starts them with the basics up to rehearsal point. That way, when they ARE assigned and working with you, you can teach them more in depth, and provide the second part of the guide. The second part would include:
During rehearsals, blocking (maybe this would be in the first parter), prompt book, preparing for tech, etc.
The second part could be subject matter that is dealt with later on in the process, and therefore, dealt with later on once they get into the swing of things and a handle on the first part...
Does this make sense?
That's my suggestion. I've trained stage managers before, and it wasn't until AFTER I had done so that I thought "hm...it would have been a good idea to break it down a little better" so this is my thought on that.