UX is a fancy name put onto practical things by consultants so they can charge for them. In practice there are three main points.
First, there is the matter of knowing what is common to all people. How the brain works, how the eyes scan, how people tick, what makes a screen understandable and so on. Principles of clarity apply regardless of what the domain is. Tufte’s principles for example apply here.
Second, you need to know what is special about your users. It’s not their brains or perception – that’s the same in everybody. What makes your users different are their habits and the job they are trying to do. You need to understand their purpose for using your product, how they talk about it, their points of reference and so on. Technically this is called ‘domain knowledge’. It’s most useful to think about the jobs individual users are trying to do.
Lastly you need empathy, the ability to see these points from the user’s perspective. You have to see it through their eyes in order to make the right decisions. You can use friendship as an example. You usually know what your friends like or don’t like. So try to become friends with your users. If that’s too hard, make friends with someone else in the same role.