How does one go about creating a truly alien culture?

Mental differences are especially important. I can do the physical things by just looking at real life aliens (like octopuses). Ex: how do you think octopuses see the world? We can’t actually think like an octopus, but we can still imagine what it might be like.<br><br>But how do you get into the head of aliens that you made up? Do you just go off of instinct?<br><br>And how do you go about making those aliens *truly* alien, not just humans in alien costumes?<br><br>The Octopus System and the Raven Tower by Ann Leckie are good examples of what I’m looking for.