first, the reduction of gaseous oxygen to aqueous hydrogen peroxide is:
O2 (g) → H2O2 (l)
when this reaction in acidic aqueous solution, and we need to balance H on both sides so we will use H+ on the left side.
O2(g) + H+(aq) → H2O2(l)
now we have 1 of H atom on the left side and 2 in the right side so we will but 2H+ on the left side instead of H+:
O2(g) + 2H+(aq) → H2O2(l)
now we need to balance the 2 positive charge of 2H+ so, we will put 2 e- on the left side:
O2(g) + 2H+(aq) + 2 e- → H2O2(l)
and this is the final balanced equation which has equal no. of atoms on both sides:
2 of O atom, 2 of H atom on both sides.