1) Define target market by industry (x), country (y), and size
2) Find out the job titles (z) in these companies that make decisions about buying
3) For each of the x * y * z = n segments, locate 5-10 people to interview
4) Conduct the interviews
5) Analyze the interviews for key pain points, needs, willingness to buy
6) Based on patterns, reduce the number. For example, 12 segments can become 3 personas.
7) Give your personas name and picture. Write descriptions that include pain points, needs, and willingness to buy. Add direct quotes to support. Done.