A reductant (or reducing agent) is the species that is oxidised. The reductant is the species that reduces another substance.
An oxidant (or oxidising agent) is the species that is reduced. The oxidant is the species that oxidises another substance.
Use oxidation numbers to decide which species is oxidised and which species is reduced.
Balance atoms that have been oxidised or reduced (this is usually atoms that are not oxygen and hydrogen).
Balance oxygen atoms by adding water to the opposite side.
Balance hydrogen atoms by adding hydrogen ions to the opposite side.
Balance charges by adding electrons to the more positive side, since electrons are negatively charged.
Combining the two half-equations
To combine the two half-equations manipulate the equations so that they have the same number of electrons. This may involve multiplying one or both equations by a factor.
Combine the two equations by putting reactants from both half-equations to the left of the arrow, and products to the right of the arrow, and cancel out any species common to both sides