For POP3, use pop.fastmail.com on port 995 with SSL. You will need to use an App password (see
https://www.fastmail.com/help/clients/apppassword.html).
Basic forwarding is not complicated, but issues can sometimes arise, especially when receiving from mailing lists. If you set up a forwarding Rule in the past, and encountered problems, perhaps you could explain the issue before we try to provide advice.