On a desktop or laptop device towards the top right of the page, you should have the "Existing user? Sign In" drop-down (just to the left of the orange "Membership: Join Online" box).
Click the "Existing user? Sign In" drop-down and then fill in your username and password. (Note: Your username and password for the BN main website will be the same as your username and password for the BN membership system - that is, if you have an account on the BN membership system.)
If you have forgotten your username or password then please refer to this guide: https://www.bn.org.uk/guides/not-logged-in/what-is-my-username-and-password-members-only-r10/
If you find any errors or want to provide other feedback about this guide then please send email to email@example.com. Thank-you!