I suspect that what you mean is that you have a $Z_2$ symmetric theory $\mathcal{L} = -\frac12 (\partial\phi)^2+\frac12\mu^2\phi^2-\lambda\phi^4 $ and you would like to compute the Lagrangian appropriate for fluctuations about the phase where the $Z_2$ is spontaneously broken. You would want to write $\phi(x,t)=\bar\Phi + \varphi(x,t) $ where $\bar\Phi$ is the minimum of the potential $V\left(\phi\right) = - \frac12\mu^2\phi^2 +\lambda\phi^4 $. You can then plug this decomposition into $\mathcal L$, and keep all the terms that depend on $\varphi$. It's just algebra.