Answer
This is done by using a changing the ratio of methane to chlorine.
Work Step by Step
In order to get more $CH_3Cl$ in the products, we must keep the ratio of methane to chlorine high in the reaction mixture.
If we want a good yield of $CCl_4$, we need to get maximum replacement of the hydrogen atoms by chlorine. This can be done by keeping a high ratio of chlorine to methane in the reaction mixture.
