I always explain correct use of mirrors and the need for a blindspot check at the end of the controls lesson and immediately before the demonstration drive. I will emphasise all this during the demonstration amongst many other topics as they inevitably arise.
Having arrived a few minutes later at an industrial estate I have a conversation about what has been said and done so far. Then the candidate moves to the driver’s seat and gets comfortable. The very next thing I do is to, with the driver’s window open, stand at the side of truck on the driver’s side and ask if I can be seen in the mirror. Of course I can. Then I step towards the centre of the road a yard or so. At this point they can generally see my shoulder. A small move further and I cant be seen in the mirror. I then point out that I am a 16 year old (use your imagination here!) on a scooter just passing the truck as he is about to move away. To drum the point home, I then walk forward about 8 feet in front of the truck and indicate that this is the point at which he kills me because he couldn’t be bothered to look. I rarely have to repeat this instruction.
On another point, some examiners take different views on this. And this is why it’s perfectly possible for a “notional” check to pass a test (sadly). I have found that, almost without exception, more recently trained examiners are expecting the check to be done properly ie a serious look behind. I totally support this and it causes us no difficulty at all.
I think one of the biggest problems that some instructors have (and I’m NOT including the OP in this as I probably don’t know him/her) is that an instruction will be given eg “check over your shoulder” with little or no explanation as to WHY this is needed. If I’m learning a new skill I need to know why I need to perform any particular action. Otherwise it doesn’t sink in. And I’m not that unusual.
Hope this helps, Pete 