Being a Fixit Coach is easy: at the most basic level all you do is provide participants 1) guidance with tools and 2) encouragement to investigate their broken item. Be a listener more than a repairer; let them talk about how they used it and how it broke: that often provides valuable hints as to what's wrong. See how much you can do without actually handling the item: empower the item's owner to do the work. Move among items and consult with others to cross-train and maximize transfer of knowledge.
(For more on Fixit Clinic: www.fixitclinic.org)