To read a verier: Look at the left tick mark on the bottom moveable vernier scale and read the value on the top scale, approximating the reading to better than the distance between the tick marks. Ie, if the left tick on the vernier lies between 3.7 and 3.8, you can approximate the actual reading by trying to get another decimal point. It will not be very accurate, but you can at least guess whether it is closer to 3.7 or 3.8 Lets say you guess it to be 3.72 (if you guess it to be 0, then write down that 0-- ie it would be 3.70) Now look at the verier tick marks (bottom scale). and see which of the verier tick marks line up with the tick marks on the top scale. In the example, it is the 4th vernier (bottom scale) tick mark which lines up with one of the tick marks on the top scale. The value of the tick mark on the top scale that lines up does not matter and is ignored. However the value of the bottom tick mark does matter. In this case it is the third one. The left most vernier tick is labeled as 0, the next 1, and so on up to 10 (you never use the 10). So in our case the reading is 3.
Now replace the last decimal place in the reading by this value. In this case we replace the last 2 in 3.72 with that 3 in the vernier scale that lined up. Ie, the reading is 3.73. That you did not guess the right value at first is fine. It is just there to give you a rough idea. The verier lining up gives you the better value. Note that your rough idea should be at least close to the one you get from the vernier. Thus if you guess, 3.78 and the vernier say 1, then something is wrong. Figure out what the problem is before committing to your answer.
What if none of the ticks on the vernier scale line up? You look for the one that most nearly lines up. (One of the vernier ticks will lie just to the right of the above tick mark, and the vernier tick lies to the left of the one above. That means that the value lies between the two vernier tick marks. Use the one of the two that is closest to lining up.
The above assumes that there are 11 vernier mark (0,1,2,....,10) The accuracy of the vernier reading is 1/(N-1) were N is the number of vernier tick marks. Thus if there are 20, the accuracy would be 1/20, and the values would be 00 05 10 15 20 25 ....95 100. If there were only 6 vernier marks, the accuracy would be 1/5, with values 0 2 4 6 8 10