There are tons of ways of doing it, but all of them necessarily cost more energy than you extracted from the fossil fuel. So, from an energy economics point of view, first we stop extracting the so-called "unburnable" fossil fuels - end conventional and shale oil exploration and phase out their extraction. Then, with our spare renewable energy, we can start condensing fuel out of the atmosphere; there are several processes for this, including one the US Navy is pioneering for at-sea refuelling of planes on nuclear carriers.