Sorry, the only (and best) advice I can give is a) make reeeaalllll sure you've thought through the liability/insurance aspects of this, even if the kid is a relative (insurance companies can do whatever they want...); and b) if you have to ask, IME it typically means that maybe it is not such a good idea. (As opposed to having worked under a good instructor for long enough, as student or working-student or assistant, that you can use what they do as a basic template, gradually making it your own as time goes by)
If you're bound and determined to do it anyhow, then
a) back to the liability/insurance thing, really really, THINK about it... remembering it is not always the person's own choice whether to sue or not (unless the laws have changed a whole lot since I moved away 8 yrs ago)...
b) start by ensuring that the horse is totally suitable and the tack correctly fitted to both horse and rider
c) rider needs to be wearing well-fitted hard hat and some kind of shoes with heels, also preferably something on their lower legs that will not cause rubs (you can bandage over jeans with a track bandage if necessary)
d) start by teaching (or testing, if this is not a total novice) the rider the ability to lead the horse on the ground including turning and stopping; then do mounting AND DISMOUNTING before proceeding to any kind of actual riding lessons. Use a mounting block, like a SAFE SECURE one. And when dismounting, insist they kick BOTH feet free of the stirrups as soon as they get ready to dismount -- none of this 'throw right leg over the horse's back and *then* drop the left stirrup".
e) teach only, and I really do mean ONLY, in a smallish, safely-fenced, un-junky area with decent footing, where there will not be sudden excitements like kids barrelling around the corner of the barn on motorbikes or dogs racing by. By safely fenced I mean you should ask yourself, if the horse (no matter how calm you are sure he is) should bolt and stop short, dumping the rider headfirst or at an angle into the fence, is the rider going to get tangled in wire or impaled on a t-post.
f) if the horse is questionable, or the tack not fitted correctly to him or to the rider, or if an appropriate ring (as per above) is not available, DO NOT AGREE TO DO THE LESSONS. I speak from both experience and observation of other peoples' experience when I say that you have no idea how much can go how wrong and with what consequences.
g) some people will say you should always start a beginner on the longe line. I'm less convinced of this but if it is a REALLY quiet horse and quiet area, and the rider has a poor natural seat and needs to be able to concentrate on that before they can deal with actually guiding the horse, then a longe line (or leadrope as you walk along) is useful.
Beyond that, if it's a beginner you can do the standard steering exercises at a walk with cones and objects on fenceposts and such; and jog along going 'up down up down' when they're ready to trot; and all the usual stuff. You're not going to get from a BOOK, though, how to do this safely, or how to judge when it is time (versus premature) to up the ante, or how to give useful rather than counterproductive advice/corrections/help, or how to keep the rider's interest. It's a mileage thing, and I really honestly believe you have to have a strong imprint in your head of someone who was good at it, to be able to get off to a strong start yourself (at teaching).
Good luck, have fun, think carefully about this,
Pat
