Instructional steps to follow
Sign in to your Yahoo! email account as you would normally. Click "Options," then "Mail Options" on your Yahoo! dashboard.
Click "Accounts" on the left-hand sidebar. Choose "Add or edit an account" from the main window.
"Account Information" on the next screen. The system prompts you to
sign in again with your password. This brings up the "Yahoo! Account
Scroll down a bit and select "Manage your Yahoo! aliases." A new screen appears describing the purpose of having a Yahoo! alias.
"Add an alias" and type in a new name. This name has to be unique--no
other Yahoo! user can have it. You may have to try a few names before
the system accepts one.
Take note of your new alias name and click "Close."
Log out of your Yahoo! account. Log back in to your account with the new alias name.