The plan involves first launching an uncrewed "Earth Return Vehicle" directly from Earth's surface to Mars using a heavy-lift booster (no bigger than the Saturn V used for the Apollo missions), containing a supply of hydrogen, a chemical plant and a small nuclear reactor. Once on Mars, a relatively simple set of chemical reactions would convert the hydrogen, and carbon dioxide from the Martian atmosphere, into methane/oxygen propellant to return the ERV to Earth, as well as additional supplies of oxygen and methane.
A year later, once the propellant production for the ERV was complete, the crew of four would be launched direct to Mars by another heavy-lift booster. Artificial gravity would be generated for the trip by tying the spent upper stage of the booster to the crew habitat, and starting the system rotating. Once at Mars, the useless spent upper stage would be jettisoned. The crew would aerobrake into Mars orbit and land near the ERV. Their habitat/landing craft would also contain a rover powered by the excess methane/oxygen propellant generated by the chemical plant.
The crew would spend a year on Mars, using the rover to explore extensively.
To return, they use the ERV, leaving the habitat for the possible use of subsequent explorers. The propulsion stage of the ERV would be used as a counterbalance to generate artificial gravity for the trip back.
A design reference mission based on Zubrin's proposal was costed by NASA's Jet Propulsion Laboratory, who estimated the cost at around $50 billion for three missions. Zubrin claims that the private sector could do the missions for around $7 billion.