Answer: By balancing of positive and negative charge.
Step-by-step explanation: For formation of a neutral ionic compound, the charges on cation and anion must be balanced. The cation is formed by loss of electrons by metals and anions are formed by gain of electrons by non metals.
The metal has an oxidation state of positive called as cation and non metal is an anion with oxidation state of negative. Thus they combine and their oxidation states are exchanged and written in simplest whole number ratios to give neutral compound.
The cations and anions being oppositely charged attract each other through strong coloumbic forces and form an ionic bond.
For example: Neutral sodium chloride is formed by combination of
and
.