How do we exact measurement by Vernier Calliper and what is principle of vernier calliper?

First close the jaws of the vernier callipers and note down the zero error if any
2nd put the object to be measured inside the jaws of a vernier scale, then note down the main scale reading, which is the reading in the main scale corresponding to the zeroth mark in the vernier scale and note it down.
3rd. then take a magnifying glass and find the mark on vernier scale exactly coinciding with any of the main scale. Note the number of the mark coinciding, this is known as vernier scale reading.

4th. Multiply the vernier scale reading with least count of the callipers and add with the main scale reading to get the accurate reading. If zero error is there adjust it accordingly and you get the final form of the reading.

The principle of Vernier Callipers 

There is a theorem in advanced mathematics that guarantees the coinciding of one Vernier scale division with one of the main scale division. But discussion of that theorem is well beyond the scope of 12th standard.

However, evaluating the least count of vernier  callipers exists.

N-1 number of main scale divisions coincides exactly with N Vernier scale divisions thus 1 vernier scale division = (N-1)/N main scale division.

Now the least gap between division of main scale and vernier scale becomes 1-(N-1)/N = 1/N = LC
Now from the above theorem it holds that at last one division of vernier scale must coincide with the main scale generally vernier scale has 10 divisions thus LC becomes 1/N = 1/10 mm