Name: Your Name (What ever you want)
Email: Your username
Password: Your yahoo.co.jp account password
Description: Yahoo! Japan
Click Next Button
Hostname: imap.mail.yahoo.co.jp
Username: Your username
Password: Your yahoo.co.jp account password
Hostname: apple.smtp.mail.yahoo.co.jp
Username: Your username
Password: Your yahoo.co.jp account password
Click Save Button
Use SSL: Yes
Authentication: Password
Server Port: 993
Use SSL: Yes
Authentication: Password
Server Port: 465
You can login yahoo.co.jp and Signup for yahoo.co.jp new account on your iPhone from here.