Although not really a substitute for practical assistance from professional driving instructors, supervising a learner driver for private practice is a good way to save some money, by cutting down the number of driving lessons required to pass the driving test.
Number 1: Ensure the car is suitably insured for the new driver! This should probably go without saying.
Number 2: Plan WELL ahead - bear in mind that not only will the person who is driving the car have to do what you instruct, but you will potentially have to notice the hazard, put a sentence together (which might include a semi-complicated instruction), the driver will have to understand, and then do what ever it is you said.
This can be easily overlooked, as when you, yourself are driving, it can only take a moment to react.
Number 3: Do not shout! Bear in mind that for a learner driver to react best to what you are saying, they will need to be reasonably calm - and shouting will probably make matters much worse.
Number 4: Sort out your own driving! It is much harder to correct another persons driving behaviour if you own is a mess.
Number 5: Keep the focus on good, safe driving, as opposed to 'do this to pass your test' or 'don't do this because an examiner...
'.
This sort of comment will generally do more to encourage the pupil to 'act better' rather than really improve.
Driving instructors will generally have gone through training covering aspects such as these, involving much expert input and many hours of practicing basics like these as well as other aspects like aspects such as teaching techniques and of course the required driving standards for themselves and the candidates who will be taking their driving test.
It can be a difficult job contending with comments like "But my driving instructor says...
" - but if done properly, private practice can knock several valuable hours off the number of hours required to learn to drive.