If you go to MittRomney.com/JobsPlan  you see an ad in which Romney says:
Let me tell you how I will create 12 million jobs ... First, my energy independence policy means more than 3 million new jobs, many of them in manufacturing. My tax reform plan to lower rates for the middle class and for small business creates 7 million more. And expanding trade, cracking down on China, and improving job training takes us to over 12 million new jobs.
While 3 + 7 + 2 = 12, that's about all that adds up here.
Those 3 million energy jobs?
They are not Romney energy jobs. They are Obama energy jobs.
The Fortune magazine report "Inside Romney's fuzzy energy jobs math"  finds:
The Romney campaign's energy job growth projections are primarily based on a March 2012 report from Citigroup that details the potential growth in domestic oil and gas production and the impact such growth would have on the economy as a whole. The report estimates that increased domestic oil and natural gas production could create more than 3.6 million jobs by 2020.
These estimates, however, do not depend on loosening regulations or handing over control of federal lands to the states, the only energy policy prescriptions that the Romney campaign has offered so far. In fact, Citigroup's projections assume that the U.S. would keep its current energy policies, according to Ed Morse, managing director and head of global commodities research at Citigroup Global Markets and the report's lead writer.
In fact, Citigroup's report refers to current energy regulations as "benevolent" and describes them as a necessary factor in maintaining the stability of the markets.
Those 7 million jobs from tax cuts?
National Journal reported  that's based on one study from Rice University professor John Diamond, who was directly asked by the Romney campaign to score the plan. But, since the campaign gave him almost no details, he couldn't really do it:
He couldn’t model the full Romney plan, because the Republican’s campaign refuses to identify, even to him, which tax deductions and credits they would cut to make the plan revenue-neutral. “They didn’t give me a whole lot of guidance,” he said.
So Diamond filled in the blanks with largely generic assumptions and noted in his analysis that the specifics could change his results. He also assumed that Romney – contrary to his stated campaign position – would allow the Bush tax cuts to expire and then impose his 20 percent across the board rate reduction from there.
Diamond also wasn't able to rebut the Tax Policy Center's conclusion that Romney's tax cut plan -- if he followed through on his pledge to make it revenue-neutral by ending various deductions -- would inevitably raise taxes on the middle class:
Diamond didn’t analyze how the plan would affect the distribution of the tax burden across income levels. In the interview, after cautioning that he had not run the numbers, Diamond considered whether it would be possible to maintain the existing progressivity of the code under Romney’s plan.
If you don’t estimate the growth effects of reform, he said, “no, it’s not possible,” to maintain current progressivity. “And I think even with substantial economic growth, it will be difficult.”
In any event, to believe that another around of giant tax cuts skewed the wealthy will unleash job growth is to believe that the first round from President George W. Bush worked. In reality, it led to the worst jobs record on record. 
Those 2 millions jobs from new trade policies?
Yet again a number from a report that does not actually analyze Romney's plans.
As the Washington Post reports today , "This figure comes from a 2011 International Trade Commission report, which estimated that there could be a gain of 2.1 million jobs if China stopped infringing on U.S. intellectual property rights ... the report does not examine any of Romney’s proposed policies."
I'd further note that while Romney is trying to sell himself today as a fair trade enforcer, his campaign webpage on trade is a literal ode to President George W. Bush's "open market" trade policies:  "George W. Bush successfully negotiated eleven FTAs ["free" trade agreements], encompassing sixteen countries ... Economists estimate that the agreements have led to the creation of 5.4 million new American jobs and support a total of nearly 18 million jobs."
That stat is another flimsy number. It's not an estimate from all "economists." It's a estimate from two economists who wrote a report for the corporate lobbyists at the U.S. Chamber of Commerce.  The Economic Policy Institue concluded the opposite, that Bush's trade policy was a net job loser. 
His recent book, "No Apology" is also critical of President Obama's efforts to enforce trade laws with China, deeming it "protectionism" . Clearly, Romney's overall trade strategy would be an extension of Bush's hands-off "open markets" policy, not a new era of more aggressive enforcement.
Finally, Romney engages in all of this fuzzy math just to eke his way to 12 million jobs ... a number that the economists at Moody's already project will be reached by 2016 no matter who is president. 
That's a lot of lying just to run in place.