Ask them various questions in order to get quality data about their demographics, interests, social life, financial information, influences on decision-making, browsing habits and content preferences, their buying incentives, responsibilities, daily activities, challenges and, last but not least, their pain points. Such interviews and feedback can give you in-depth information about your customers, their preferences, and the kind of marketing message that will appeal to them. You can help yourself using predefined persona templates for that part.