Determine the change in oxidation number per atom for the elements undergoing a change in oxidation number. Draw a large bracket from the element in reactant side to that in product side, and write down the increase (+) and decrease (-) in oxidation number at the middle of the bracket. The increase in the oxidation number by the oxidized species means oxidation reaction and the decrease by the reduced species means reduction reaction.

In the case of Disproportionation Reaction, where the same element is both oxidized and reduced, such species is written twice on its side. See Example II here.