How would you balance H2+O2-->H2O?
The same way as I would balance my change after a cash transaction.
Mass is always conserved. Are the atoms on the left hand side equal to the atoms on the RHS? If no, you've still got work to do. If yes, then you are in principle correct.
You do this balancing, this stoichiometry, every time you make a cash transaction or an electronic transfer. Debit must equal credit! If not, some blighter has made a mistake.