Look for the changes in oxidation state – there should be only one element changing in a half-equation, two or more in a full equation. O and H probably don’t change.
Balance any oxygens by adding water
Assuming acid conditions, balance any hydrogens by adding H+
Balance oxidation numbers with electrons
E.g. MnO4- MnO2 becomes
MnO4- + 4H+ + 3e- MnO2 + 2H2O