To balance a chemical equation, you want the same amount of elements to equal the same on both sides.
Step1. Write equation out
[CH4 + Cl2 ---> CCl4 + HCl]
C:1; H:4; Cl:2 C:1; H:1; CL:5 /// Cl = 5 since 4Cl + 1Cl
The Carbon element is balanced, but Hydrogen isn't.
So to balance it we will add a coefficient behind HCl, so now
[CH4 + Cl2 ------> CCl4 + 4HCL]
C:1; H:4; Cl:2 C:1; H:4; CL:8 ///// Carbon and hydrogen are balanced, but now Chlorine is not. Now we balance that element by addind a coefficiant behind CL2////
[CH4 + 4Cl2 -----> CCl4 + 4HCl]
C:1; H:4; Cl:8 C:1; H:4; CL:8 ///// So now that we added a 4*Cl2, it equals to Cl:8. So now what most people want to see is if every element is at its lowest balance, so we see if we can any coefficient lower. Just like simplifying if possible.