We describe in detail an approach to the numerical modeling of second-order nonlinear-optical parametric processes in guided-wave structures. For numerical solution of the nonlinear evolutional equations, we choose a finite-element method in conjunction with the Crank-Nicolson method. An extreme enhancement of computational efficiencies is possible by means of a split-step procedure. Through a variety of numerical examples for Cerenkov radiation and quasi phase matching, we demonstrate that the present numerical solution method is highly useful for theoretically predicting optimal device configurations and for designing efficient guided-wave frequency converters.