Planning an alien culture requires thought about the components of various earth cultures. Actually, it requires the author to choose an earth culture that will be close to what he or she envisions the alien culture will be.
As much as we would like to believe we have original thoughts, it would truly be difficult to conceive a totally alien culture. And if it was that different from earth cultures, how would earthlings communicate? How would we find common ground? Even now, we find it nearly impossible to find common ground between earth cultures.
As strange as it may sound, I usually first decide what the physical environment will be like. Is it a desert? Or a rain forest? Or an island paradise with hidden serpents? That physical environment will ultimately determine what is valued by that culture. Water? Land? Sea life?
What do the inhabitants eat? How is it obtained? Are they hunters or farmers? Or an odd combination? Maybe they eat food manufactured by overseers. Maybe they're vegetarians.
What is the technology level? If they are a peaceful society, how is that peace maintained? What does their justice system look like? If they are a warrior society, what type of weaponry do they have?
What about the family composition? It is a patrilineal society or a matrilineal society? Are the family units nuclear or extended?
In historical novels--particularly romances--there is a hallmark authors strive to pass when they write. They strive to write a book that is more than a wallpaper setting. The same is true for novels set in an alien setting. It cannot just be a story with some alien names and a couple of alien costumes, yet set in a background with earth values and technology.
The author has to plan on full immersion in their alien culture. If they are going to have earth values, then they must account for that in some way. Who is to say that aliens would practice monogamy or even have such a concept? Would they be limited to one spouse? Or would they even have a concept of commitment? Perhaps they would have a culture that more closely resembled that of a house cat.
What do their shelters look like? Or do they even need shelters? Perhaps they have no concept of privacy.
Do they make music? Do they create art? Do they have a written language? Do they have such abstract concepts as time and afterlife? Simple everyday phrases such as "just a minute" and "time is of the essence" might not work in dialogue in that case. If their concepts do include time, how do they mark time?
The process I've just illustrated is usually called world building. The best novels set on other worlds have extensive world building. The story (boy meet girl--or whatever it might be) could take place in almost any setting. What will make it different is how detailed the setting is and how the characters confront and deal with that setting.
That's my take on world building. What do you think? How would your planning be different?