Being a Fixit Coach is easy: at the most basic level all you do is 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 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 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)