This paper presents a practical method for allocating inertia and damping in power systems integrated with renewable energy sources (RES). The frequency response model of the power system is first established, with distinct modeling of the dynamics of synchronous generators (SGs) and three types of RES units. A state-space model, which describes the relationship between frequency response and disturbances, is derived through the disturbance allocation process. A fitting method for frequency and power responses is proposed based on modal analysis. Analytical formulas for frequency stability indices (FSIs) and power indices are derived. Furthermore, applying matrix perturbation theory, an iterative optimization method for inertia and damping allocation of RES units is proposed and validated through simulation cases using the modified IEEE 39-bus model.