Just head to your Bulb account on bulb.co.uk and sign in using the same email address you used when you signed up with Bulb, and your password.
If you don't know your password or it's tricky to type, you'll see the option to ‘email me a link to sign in instantly’. This will send an email to the address attached to your account. Clicking the button in this email will sign you in. Emails can sometimes end up in your spam folder so be sure to check there.
Remember to keep the link in the email private. Although it expires within 24 hours, anyone with the link during this time will be able to access your account.
If you're having any trouble with these options you can also reset your password.