It's really easy! Just follow these steps:

1. Click Options in the upper-right corner of your Mail page and select Mail Options.
2. From the list on the left, click Accounts.
3. The second half of this page is called "Extra Mail Address." Click Get Started and follow the instructions to create an extra mail address.

Note: When you create an extra mail address, it appears in the list of Yahoo! Mail accounts shown on this page. To help prevent spamming, an extra address can only be changed twice in one year.