1Count the atoms of each element in the reactants and the products.
2.Use coefficients; place them in front of the compounds as needed.
H2 + O2 -> H2O
To balance this the unbalanced oxygen molecules, we add 2 in front of the product on R.H.S means in products . We get,H2 + O2 -> (2) H2O
Now, we have 2 Oxygen's but 4 Hydrogen's on the R.H.S.
In order to balance the same, we add an another 2 in front of Hydrogen on the L.H.S
(2) H2 + O2 -> (2) H2O
Now, the equation is balanced with 4 Hydrogen's (H) and 4 Oxygen's on both sides.
You need to practice this In starting it looks difficult but once the concept is clear in your mind you can easily do it