Most important is how you relate. Unfortunately, it’s mostly don’ts.
Don’t expect your relationship to accumulate trust or predictability. Don’t assume that what happened in previous sessions will carry over. Forget Carl Rodgers and Person-Centered Therapy. Think Bill Murray in GroundHog Day.
Don’t use relationship language between you and the client. Don’t tell them they can trust you or that you care about them. Talk is meaningless; only your behavior counts. Don’t make promises you can’t make happen within five minutes (the lone exception is, if they bring up imminent harm to self or others, you can promise an eval from a mental health professional).
Counselors have dealt with clients’ delusions in a variety of ways. From don’t talk like that, it’s crazy, to discussing their delusional universe, at length. Good luck with that. Given your task, it will probably go nowhere useful.
What to do? It’s not that complicated. Do what you always do with clients. Whatever symptoms your clients are having (delusions included), how are those symptoms impeding what they want? What they might want: to stay out of the hospital; to avoid being evaluated by mental health professionals for involuntary treatment; to see a mental health clinician less often. Whatever it is, that’s all you’re responsible for helping with. Occasionally, someone will want more. Great, help them with that.