Step 1: Where Do They Come From?
Find a general biome that fits what you envision for this culture. If appropriate, make up your own. You want to focus on how plentiful the water is and where it is, what food sources there are, and what natural resources (wood, iron, reeds, etc.) are available. You’ll also want to look into natural structures like caves or cliffs, and common weather phenomenon like hurricanes or droughts.
If you’re writing a premodern culture with few outside influences, you could stop here, since location pretty much gives you everything you could want. The local vegetation and weather patterns will dictate how they build houses. The natural phenomenon will be explained by religion. The availability of water and food/arable soil/animals that could be domesticated will determine if they are nomadic or not. Their natural resources will determine how quickly technology progresses.