You don't have to be the best pilot in your club to be a good instructor. You have to be able to fly a trainer-type plane safely, to get it out of trouble if it suddenly heads toward the ground or the flight line, and to understand the basics of what you're doing, like using throttle to control altitude on descent, crabbing into the wind for crosswind landings, and knowing why a plane in flight never "feels" a steady wind, though its ground track will be affected by one (which will confuse some pilots). And you need to be patient and willing to put in the time.

For me, the hardest thing at first was learning to keep my head in the game when the student was flying: being constantly aware of which way the plane is turning, for instance. It's not like just watching another person fly; you've got to be ready to take over right away.