Let h_n the homogenous symmetric function of degree n. You can look at it as the generating function of weakly increasing sequences of integers of length n. Let m_I be the monomial symmetric function. You can look at it as the generating function of weakly increasing sequeces, whose weight is a permutation of I. We will give an algorithm to compute the decomposition of the plethysm S_n[m_I] as a sum of monomial symmetric functions.