Being a Fixit Coach is easy: simply provide participants with 1) encouragement to investigate their broken item and 2) guidance with tools. Be a listener more than a repairer; let them talk about how they used the item 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 troubleshooting and disassembly. Move among items and consult with others to cross-train and maximize transfer of knowledge for everybody -- including you!
(For more on Fixit Clinic: www.fixitclinic.org, https://www.facebook.com/FixitClinic/, https://plus.google.com/+FixitClinic)