In Europe....
EASA EU1178/2011 FCL.900
"(a) General. A person shall only carry out:
1) flight Instruction in aircraft when he/she holds:
i) a pilot licence issued or accepted in accordance with this Regulation;
ii) an instructor certificate appropriate to the instruction given, issued in accordance with this subpart."
Not only should you be properly qualified, you should also be operating under a suitable flying school approval E.g. an ATO and at a suitable training aerodrome.
If you have an accident, the insurers will simply laugh at you and heaven help you if someone else is killed - you'll go to prison.
If you have not trained to be an instructor, you don't know what you don't know! Ask any newly qualified instructor what they have learned in their first six months and they will invariably tell you about someone doing something they had not expected.
As Ascend says it's not a good idea. I know of at least two UK accidents where private owners have let someone have a go and crashed of a result. It's this sort of irresponsibility that puts our insurance premiums up. Best don't do it!