1.Preliminary definition of what primary user groups are relevant. Goals, roles, demographics, expertise, are all good initial attributes you can use to define user groups.
2.You perform user research
3.Analyze Your results.
4.Then requires a choice in terms of methodologies for investigating said groups. Common user research methodologies are interviews, surveys, focus groups, heuristic evaluation and shadowing.
5.Turn them into design specifications you can move forward to help you shape your goals and process.