Many papers have been published on the theory of magnetic insulation and the use of Z_flow analysis of magnetically insulated transmission lines (MITLs). We describe herein a novel design process using the circuit code SCREAMER for a real-world MITL for z-pinch loads based on the Z_flow model of magnetic insulation. In particular, we design a 15-TW, 10-MA, 100-ns double-disk transmission line using only circuit modeling tools and Z_flow analysis of the MITL. Critical issues such as current loss to the anode during the setup of magnetic insulation and the transition from a non-emitting vacuum power feed to an MITL play a large role in the MITL design. This very rapid design process allows us for the first time to explore innovative MITL designs such as variable-impedance MITLs that provide a significantly lower total inductance and improved energy delivery to the load. The tedious process of modeling the final MITL design with highly resolved 2D and 3D electromagnetic particle-in-cell codes occurs as a validation step, not as part of the design process.