I myself just started instructing.  I was unsure as you are till I had a talk with my current mentor about it.  He told me he saw me fly and that I was capable of doing the instructor thing.  I had already started doing some ground work helping the pther instructors so the teaching part was there.  I was a bit apprehensive to fly someone elses plane.  Well we had an opertunity to assist a school in doing buddy box flights for the students and it was so much easier than I thought.  I just kept the plane where I was comfortable then gave the student the control.  I think this is what really helped my confidence in buddy boxing.  There was no lesson or teaching it was just let them goof off with the plane then take it when it was not to your comfort.  One older student was actually taking to lessons and landed on my first day of buddy boxing.  It all has to do with your comfort.  If you can take your plane and recover from adverse attitudes then you know what there is to do.  The hardest part is not panicing and work on calmly saying my plane fix it then give it back.  Explain what happened if you know and what can be done to remedy the problem.  If you remain calm then it goes a long way to the student gaining comfort with you.  I lost the rudder on the trainer when I was planning to treach a student.  It had a failed hinge and was jamming over causing a roll when jammed.  I took off and had the failure immediatly called that I was in emergency condition to the other pilots then calmly climbed high to get an idea of what to do.  I wiggle things and got the rudder to slip back into place then set up and landed right away.  The student was scared for me.  He was like man I thought that was going to crash for sure.  I just repeated what was taught to me never stop flying and altitude is your friend.  I was shaking after the landing and thought I had just bought a trainer for sure myself but the comfort from those around came from keeping my composure and keeping my lessons in kind as things happened.  Stay in your comfort and remain calm.