Representing your customers’ perspective
The customer journey map needs to represent the interactions as your customer experiences it. It often includes interactions that happen outside of your control, such as a social media interaction or a web search.
This can include interviews or ethnographies, possibly combined with surveys. Some companies bring in customers and build them interactively with internal staff. Ethnographies can create a very powerful experience, although the small sample size can create bias. Better to do the research first, then bring in your customers to build your final map.
Your different segments typically have very different customer experiences. For example one segment within a retail organisation may typically spent two hours researching the category, while another consistently spent more than six weeks doing the same, using very different tools. Imagine trying to represent these very different experiences as one.