How to apply gauss's law and its explanation

Student suffers and lot of problem during application of gauss law
Now I am going to explain how you can easily apply this law to different -2
geometry

1:- Choice of gaussian surface
2:- line of force
3:-Area vector  (imagine you are inside that geometry than count number of sides )

Now Lets take a example of cylindrical body
 which is placed horizontally along the electric line of force (electric field)

now if we look at it
-we get three surface
one curved surface 
two circular cross section surface 

Now take the flux from each surface
along curved surface flux will be  zero 

(apply on formula angle will be 90 degree )

and along the cross section surface 

In both section flux will be equal but in opposite direction
so we can conclude that sum of flux along all surface is equal to zero