From Yuku Help

Creating your Account

To manage your account, first you need to create one. Do this by clicking the sign up button found at the top of any yuku domain.

(If you have an ezboard account that you wish to transfer over, don't sign up for a new account, but go to your ezboard account control panel and use the transfer button from there. See ezboard account transfer)

After clicking signup, you'll need to fill out two pages as part of the sign up process. This is the first page:

This page sets up your account.

Your account can then have up to 5 profiles, so the next screen will set up your first one. A profile is the same as a username - this is what other people will see.

Type in the profile (username) that you want to use.

Click the "available" button to check if it's free.

If you clicked "sign up" from a community, you will notice that your profile name is going to include the boardname after it. To sign up for a global name, use the sign up link from the homepage instead of from board.

You can also specify an avatar to use on your profile. You can change this later if you wish. You can't change your profile name later, so make sure you spell it right!