This is very easy. You just fill in your desired new user name and the photo credit text that you would like to see next to your photos in print.
Your user name can be your real name or it can be a pseudonym. Allowed characters [ A-Z, a-z, 0-9, . and _ ]
The best form for the photo credit text is your first name initial, a dot and then your surname.
For example, if your name was David Attenborough then you could set your user name to davea and the photo credit text to D.Attenborough.
If the user name has been registered by someone else before then it will say so when you try to complete the registration.
Please check the web site response to your registration. Once its set you cannot view the photo credit text again and if it was set by someone else then you would be uploading photos and having them tied to somebody elses photo credit.