It is incredibly simple and relatively cheap given the costs the pollution imposes. China's mix of pollution sources is little different than the rest of the world.
Non point sources are about 40% of the emissions mix and composed mostly of vehicle emissions in the summer with some heating emissions from wood and coal in the winter. The cars they are buying for the most part have the same systems in them as they do in the US, the problem is the fuel used is incredibly dirty. This could be fixed in less than a year by requiring the same fuel standards as western countries use. The non-point winter emissions can be addressed through a combination of regulations on wood/coal burning appliances and providing better sources such as natural gas and electric heating.
The remaining 60% of emissions are all point source and can be addressed through regulation of smoke stack emissions. Almost everything seriously harmful can be scrubbed at the stack. Some emissions like nitric and sulfuric acids are harder to scrub but we have the technology today to remove the majority. The worst of the worst, PM 2.5 and other particulates are trivial to scrub, this technology has been around since the 70's. The Chinese government could mandate the installation of these systems and finance the retrofits, not only could it be done relatively quickly they could cause a massive boom in internal industries to handle this.
This isn't hard. The US, Europe and Japan have done this exact thing already. In the 60's the US had air as bad as China have now, it's what triggered our entire environmental movement, the laws and all the technology to clean it up. This is easy precisely because all the research on how to fix it has already been done and all the systems are already developed and tested. The Chinese would just need to copy the regulations and requirements the US/Europe/Japan put on their industry. It's just a matter of money, and they have plenty of money to fix this AND an authoritarian government to force it's implementation.
It's beyond silly to say this is hard. It couldn't be easier.