How do I sign up for a Yahoo! Domains account?

It depends whether you're a new customer, adding another domain to your existing account, or planning to use a domain you registered elsewhere with a Yahoo! plan.
To sign up as a new customer for a Yahoo! Domains account:

1. Visit Yahoo! Small Business Domains.
2. Enter a domain name under the "Find a domain name:" box, and click the "Search" button.
3. If your domain is available, click the "Sign Up" button. To search for a new domain, click the "Search Again" button.
4. Confirm the plan you'd like to purchase, then click "Continue."
5. Enter or confirm your payment information, and click "Continue."
6. Review your plan details, then click "Place My Order" to complete your order.

If you're an existing Small Business customer and would like to purchase an additional domain name:

1. Sign in to your account.
2. Click the "Change Plan" link for the account you wish to edit.
3. Click the "Add New Service" link in the "My Service Options" box in the upper-left corner.
4. Select Yahoo! Domains.
5. Enter your preferred domain name, then click "Search."
6. If your domain is available, click the "Sign Up" button. To search for a new domain, click the "Search Again" button.
7. Confirm the plan you'd like to purchase, then click "Continue."
8. Enter or confirm your payment information, and click "Continue."
9. Review your plan details, then click "Place My Order" to complete your order.

To use a domain you already own with a Yahoo! plan:
Note: When you use your existing domain with a Yahoo! plan, you are not transferring your domain to Yahoo! Your current provider will continue to renew your domain, and you'll pay any associated fees to that provider.

1. Visit Yahoo! Small Business Domains.
2. Click the "Already have a domain name?" link.
3. Enter your current domain name, then click "Continue."
4. Review your domain information to confirm this is your domain, then click "Continue."
5. Confirm the plan you'd like to purchase, then click "Continue."
6. Enter or confirm your payment information, and click "Continue."
7. Review your plan details, then click "Place My Order" to complete your order.

After you've placed your order, you'll need to associate your domain name with your new hosting and email services.
Source : Yahoo