This paper presents the details of development and computer implementation of Combined Field Integral Equation (CFIE) technique used in the framework of Method of Moments to address the problem of simulating fine slots appearing in grounded screens embedded in laterally shielded multilayered media. Narrow slots are encountered extensively in the new generation of ultra wideband microwave filters which use slotlines as a part of their coupling structure. The mathematical formulation is described comprehensively to allow the reader proceeding straightforwardly with the computer implementation. The considerable advantage of using magnetic currents to represent fine slots instead of using the conventional electric currents representing metallic parts is emphasized and the effects of using both current types (electric and magnetic) on the convergence of modal series representing Green’s functions is studied.