Tight-binding (TB) methods are designed to work for neutral systems. When an electron is added to or subtracted from the system, or when an external field is applied to the system, one should handle the problem of screening and charge redistribution in a self-consistent manner. Here, we have proposed a simple way to calculate the potential due to an external field and the corrected Hartree potential. A straightforward way to compute the Coulomb (Hubbard) integrals for any atom is also demonstrated. This formalism has been tested against ab initio calculations for carbon C20 and silicon Si12 clusters. Good agreement between ab initio and self-consistent TB results is found.