Once you have the basics of your world - geography, countries, people groups, - figured out, where do you go from there?
For me it seems to be a gradual process. I never really sat down and said 'Today I am going to develop the Athragani culture' or 'I will now work out all the details of the Rheuhark social system.' Those things just sort of developed themselves over time, as I worked more and more with the characters from those cultures.
Oddly enough, the culture of the Taravels, which are the race closest to us Earth Humans in my book, is the least developed of all the cultures in Reyem. For me, it is actually a bit difficult to develop a culture that has a fantasy flavor to it. Human nature is the same, regardless, so there are aspects of human culture that will carry over, even into other worlds (commerce, family relations, moral ideals, love of art and music, etc.). In order for your readers to connect with your characters, they have to be able to identify with them on some level, so you have to maintain at least some aspects of human nature in your characters. At the same time, you have to take human nature/culture and give it the flavor of other-worldliness.
So how do you do that? How do you maintain enough human nature in your characters to allow your readers to identify with them, while still giving their culture the flavor of a fantasy world?