How do I balance symbol equations?

The most important thing to remember about chemistry equations, whether these are word or symbol equations, is that you must always have the same amount of each species (element or molecule) on both sides of the reaction arrow.
If you have 2 molecules of oxygen on the left hand side of the arrow you must also have 2 molecules of oxygen on the right hand side of the arrow.
Be careful not to get confused if elements appear as part of different combinations on either side of the arrow, for example if you have oxygen as a molecule in your reactants but as part of a hydroxide group in your products. You simply need to balance the total number of oxygen, no matter what form it appears in.

If we look at the symbol equation for the neutralisation of sulfuric acid:
H2SO4 + NaOH ---> Na2SO4 + H20 (remembering that the numbers should be subscript)

​There are currently 3 atoms of hydrogen on the left hand side of the arrow, and only 2 on the right hand side of the arrow. We can see that all the hydrogen in the hydroxide group of the sodium hydroxide has become part of the water molecule.
Therefore we need to balance the equation by multiplying the amount of reactants and products until there are the same number of hydrogen atoms on both sides of the equation.
We can do this by multiplying the amount of sodium hydroxide by 2, and the amount of water by 2 to give us:

​H2SO4 + 2NaOH ---> Na2SO4 + 2H2O

​There are now 4 atoms of hydrogen on both sides of the equation, and so it is completely balanced.