Culture, if you will pardon my oversimplification, is a series of values, beliefs, and ideas held in common by a specific group of people. Everything else, from the way people dress to their particular use of language, is nothing more than an identity marker.
When writing about fictional cultures, I think the most important thing, the first step if you will, is to get the communal values and beliefs for every culture you’re writing about down pat, and to have them within easy reach for when you need them. The dress and speech can come later.
When we look at the extraordinary variety of cultures in our world, the first thing that tends to stand out is how people dress. However, if this were truly all there was to culture, there would be no culture, only fads in dressing. And we would easily be able to say, at least here in the West, that parents and their children belong to different cultures. This, however, is rarely the case.
History has shown us that when a people remain genetically the same but their values and beliefs change the culture they belonged to must die and a new one be born. This happened to Rome–genetically speaking, modern day Italians can be identical to those from the time of the empire, but, culturally speaking, they are now radically different. This also happened to the Norse countries–all of the old gods are gone, replaced by a new one, with different values. I think that this–although simplified–is worth keeping in mind, for it is when we encounter a different way of thinking and of perceiving the world that we encounter a different culture. Finally, when creating our own fictional cultures, it behooves us to have some sort of understanding of the cultures of our world, lest we risk creating unbelievable idealizations.