Balancing a chemical equation
Change only the numbers in front, never the small numbers inside a formula.
A worked example
Balance H2 + O2 -> H2O. Put a 2 in front of H2O to fix oxygen, then a 2 in front of H2 to fix hydrogen: 2H2 + O2 -> 2H2O.
The mistake people actually make
Changing H2O to H2O2 makes a different substance. Only the front numbers may change.