How do you wire a baseboard heater on a 240 volt circuit with 12/2 wire?

Does the breaker have to be a double pole or a single pole for such a job?

All the info you need is at https://www.askmehelpdesk.com/electrical-lighting/baseboard-heater-problems-help-44869.html

Yes, you need a double pole breaker, one with a contact for each leg. Don't be fooled by the twin breakers providing 2 circuits off one contact.