How to...
On the 20th anniversary of Apollo 11, 20 July 1989, President Bush Sr. asked NASA to come up with a plan to send humans to Mars. NASA came back with everything under the sun: space station as big as ISS, permanent Moon base, and Mars. Price tag was $450 billion, in 1989 dollars. Congress took one look at the price and said no way in hell! Martin-Marietta asked their engineers to come up with a plan that's sane. It would just do what the president asked: send humans to Mars. Nothing else. Robert Zubrin was one of the engineers working for them at that time. They presented a plan to NASA in June 1990, price was $20 billion for the first mission plus $2 billion for each mission thereafter. Mars and Earth align once every 26 months, so that's one mission every 26 months. They could have sent a human to Mars in 1997 or 1999.
Now that Shuttle is cancelled, NASA is building a big rocket called SLS. They could go using that. But contractors are gouging NASA, and they're going slower than a snail. They could have gone in the 1990s. Since then Martin-Marietta merged with Lockheed, it's now Lockheed-Martin. So the company interested in controlling cost (so they could get a contract) doesn't exist any more.
Robert Zubrin published the mission plan. The first book is the 1996 hard cover edition, then 1997 soft cover, then 2011, then 2013.