Honestly? Try putting the elements on each side of the reaction and count how many you have and how many you need to balance it for example:
CH4 + O2 => H2O+ CO2
Reactants:
1 C
4 H
2 O
Products:
1 C
2 H
3 O
So basically what you do now...you see that you have one carbon on each side but 4 hydrogen on recatants side and 2 hydrogen on the products side, so you need to multiply the water molecule by 2 to get the 4 hydrogens. Now because you multiplied (put coefficient 2 in front of it) the oxygen in h20 is now 2 oxygens right? So now you have 4 oxygens on the product side and 2 on the reactant side so in the reactant side, y più multiply it by 2 to get 4 oxygens.
