How do you make a WikiAnswers username via Yahoo?

Well, unfortunately you can't. Yahoo and WikiAnswers aren't linked sites, and WikiAnswers doesn't allow you to log in with a Yahoo account. If you have a certain username that you use on Yahoo that you want to use here, you can try to get that same one... on the contact us page, fill out the form to talk to the Support team, and they can look into whether a certain username is available or not.