Bascically, in a redox reaction or half equation we need to balance the oxygens, hydrogens and cahrges.
You add water to balance the Oxygens first, then H+ ions to balance the hydrogens and then electrons to balance the charges.
e.g. H2SO4 forming SO2
H2SO4 ---> SO2
Left side has more oxygens so we add two waters to the right side:
H2SO4 ---> SO2 + 2H2O
Now we have 4 hydrogens on the right but onl 2 on the left so we balance that:
H2SO4 + 2H+ ---> SO2 + 2H2O
Now you can see that they are all balanced but we still have 2+ charge on the left so to cancel that out we add 2 electrons to the left side as 2-2 = 0:
H2SO4 + 2H+ + 2e- ---> SO2 + 2H2O
Now that is our overall redox equation done.
Just remember the order:
1) Balance oxygens using water
2) Balance hydrogens using H+ ions
3) Balance charges using electrons
Hope this helps