Its required small practice while noting the reading by conventional vernier ,however by using a dial type vernier one can directly take the reading,now a days digital verniers are also available which are more precise and giving direct reading in digital form,in this small space it is lengthy & difficult to make u understand the way of reading theoretically,whereas it is more simpler if u take help of a practical person.

Reading = Main scale reading + vernier scale reading X least count

An ordinary vernier caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.

The basic steps are as follows:

1. Preparation to take the measurement, loosen the locking screw and move the slider to check if the vernier scale works properly. Before measuring, do make sure the caliper reads 0 when fully closed. If the reading is not 0, adjust the calipers jaws until you get a 0 reading. If you cant adjust the caliper, you will have to remember to add to subtract the correct offset from your final reading. Clean the measuring surfaces of both vernier caliper and the object, then you can take the measurement.

2. Close the jaws lightly on the item which you want to measure. If you are measuring something round, be sure the axis of the part is perpendicular to the caliper. ...more

Source: http://www.tresnainstrument.com/how_to_read_a_vernier_caliper.html