There are three steps to writing a net ionic equation:
Balance the chemical equation.
Write the equation in terms of all of the ions in the solution. In other words, break all of the strong electrolytes into the ions they form in aqueous solution. Make sure to indicate the formula and charge of each ion, use coefficients (numbers in front of a species) to indicate the quantity of each ion, and write (aq) after each ion to indicate it's in aqueous solution.
In the net ionic equation, all species with (s), (l), and (g) will be unchanged. Any (aq) that remain on both sides of the equation (reactants and products) can be cancelled out. These are called "spectator ions" and they don't participate in the reaction.