Identify your target user group, then carry out research to identify user scenarios and typical tasks users undertake.
Put yourself in the user's situation for a couple of hours, a day or even a week.
Carry out the tasks that they would do, in the environments where they would do them. This could mean, for example, working on a supermarket checkout or driving an unfamiliar car for a week.
Make detailed notes or keep a diary to record your thoughts.
You could...
...use empathy tools to simulate specific user characteristics. For example, wearing gloves and tinted glasses can simulate some of physical effects of old age, or a pregnancy suit with a weighted 'bump' can simulate what it would be like for a pregnant woman to use your product or service.