Computing weight $2$ modular forms of level $p^2$

For a prime $p$ we describe an algorithm for computing the Brandt matrices giving the action of the Hecke operators on the space $V$ of modular forms of weight $2$ and level $p^2$. For $p \equiv 3 \bmod 4$ we define a special Hecke stable subspace $V_0$ of $V$ which contains the space of modular forms with CM by the ring of integers of $\mathbb Q(\sqrt{-p})$ and we describe the calculation of the corresponding Brandt matrices.