# Interaction Lagrangian up to quartic order

I have a Klein-Gordon Lagrangian of scalar fields and I add an interaction term that depends only on the fields (not their derivatives). The free Lagrangian is invariant under some infinitesial symmetries and I want also the interacting one to be invariant under the same symmetries.

Are there some particular steps I have to follow in order to find the general form of the interaction term up to quartic order?