Three steps. Step 1: segment your audience into two to three clusters that differ significantly from each other. Step 2: talk to three to five real people per segment. Ask: why would you come, what do you expect, what makes it worthwhile? Ten minutes per conversation is enough.
Step 3: write out the persona with a name and two paragraphs. Use that description as a touchstone for every design decision: 'Would this work for Karin?'