While driving, do you find it difficult or a pain to turn your head in order to see the "blind spots"?
Do you want to see your "blind spots" without having to necessarily turn your head?

If you have answered "Yes" to the above, you can use the door mounted mirrors to help you. Your driving instructor perhaps showed you how none of the mirrors cover the "blind spots" and thus the name. Mine did. But that's not true!
Here's how you adjust the door mounted mirrors so that the "blind-spots" are covered in their view.

For the left blind-spot:
Sit in the driver's seat and put the side of your head against the inside of the driver's side window glass. Look into the door mounted mirror and adjust it such that the left tail light end of the car is just visible. The door mounted mirror will now reflect the left "blind-spot" when you are in the regular driving position.

For the right blind-spot:
Sit in the driver's seat and move/lean to the right in a way such that the head is located as if you were sitting in the middle between the driver's seat and the front passenger seat. Look into the passenger-side door-mounted mirror and adjust it such that the right tail light end of the car is just visible. The door mounted mirror will now reflect the right "blind-spot" when you are in the regular driving position.

Suggestion for use:
This method is not generally recommended for all situations. Use the mirrors to only to see if the "blind-spots" are clear. If a vehicle is visible in the mirror, revert to the standard method of confirming by turning your head.
This method helps in reducing the "waste" head turns that you do when the blind spots are actually clear.

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Diogenes
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The Cynic

I have found the side mirrors immensely helpful especially while changing lanes.

The rule I use though is 1/3 of the mirror shd have ur own car and the rest 2/3rds beyond it . It helps in judging the RELATIVE distance as well.

Manasvi.