I'm not sure if there's an adequately general explanation, as the methods will vary depending on what one is trying to do - synthesizing nanoparticles in aqueous solution will be different than trying to prepare them in an organic solvent, for instance. And while there are plenty of wet methods, there are some oddball syntheses out there.
Probably the most broad explanation is the same for crystal growth in general - nucleation and growth. It's not very detailed, but the diversity makes it hard to me (who is admittedly not an expert) to come up with an easy and general explanation. Researchers have tried a laundry list of methods over the years (electrochemical means, various emulsions and micellar protocols, and that's just a few of the ideas) depending on their needs and goals.
If you have a more specific question in mind, I might be able to help you with at least finding where to find a good answer.