Go to https://business.bt.com/ and log in with your BT business account email address and password. Then click on 'email'. If you use an email client instead of the web log in, be aware that the email password is not necessarily the same as the BTB account login although it may be.
Another method is to go to this link:
You need to type in your full email address like: email@example.com and your email password.
Good luck with your experience of btconnect email. Only pop access is allowed (Not IMAP) and you can't create "rules" like we can on Microsoft's free email service. Filters are OK to block some senders but you can't create filters to sort your incoming emails. All goes into inbox or junk folder. that's all you get.