Not a prison chaplain, but a prison counselor I have considered.
My one bit of advice is to remain open, and not insist people are religious or spiritual in some way in they say they aren't. In a time when you need guidance and healing, it's rather off putting to have someone insist you are something you aren't - this experience was from a hospital chaplain, who seemed cool, but she wouldn't quit insisting that I too am spiritual, just in odd ways that I don't consider spiritual, and that my spiritual health is something I need to be concerned about even though I just don't believe in the same concepts as she does.
I wish you luck in your endeavor, and hope it works out well for you. Inmates aren't really necessarily bad people, and many of them will be just fine with you if you just hear them out and refrain from judging them. They know what they did, and a moment to not be reminded can be a great thing.