Question

how do you find the net electric flux through a closed surface

Answer 1

There are two ways of finding the electric flux,
1) if the total charge that is inside the closed surface(Q) is given you can use the gausse's law i.e.
$$\Phi=\frac{Q}{\epsilon_0}$$
2)
electric flux is number of electric lines of force passing through the given surface area which is held perpendicular to the direction of electric lines of force is called electric flux.( http://en.wikipedia.org/wiki/Electric_flux)

If you know surface integral you can find the total flux as

\[\iint \limits_{S} \mathbf{E}\cdot d \mathbf{S} \]
If you dont know suface integral this can be approximately explained like this,

if you consider a single line in Electric field, the flux is the component of that Electric field line that is perpendicular to the considered plain. Likewise if we sum up all the components of the all the force lines in the electric field that is perpendicular to the plane we can find the flux going through the plane. If you find a special case where the electric field is having a constant |E| and a direction and the direction is perpendicular to the considered plane, all you have to do is multiply |E| with the area of the plane.