The cascade backward-wave amplifier (CBWA) has gained recent interest as amplifier at terahertz frequencies. The backward-wave regime permits the design of slow wave structures (SWSs) with longer period and geometrical dimensions, for a given frequency, that better match the fabrication process capabilities. The complexity of the CBWA and the use of the double corrugated waveguide as SWS to operate at terahertz frequencies require a simulation strategy to properly model the amplifier behavior and to reduce the relevant number of time-consuming particle-in-cell simulations required for a proper design. A methodology for the fast design of CBWAs is presented.