Yahoo email forwarding is all or nothing and doesn't work with existing email sitting in your mailbox. If you want to automatically forward specific yahoo emails or existing ones, we recommend transferring your Yahoo Mail inbox to a Gmail one.
To turn on mail forwarding in Yahoo mail:
Click More Settings
Click the primary mailbox account.
Under Forwarding, enter the Forwarding Address
Log in to your forwarding address and follow the instructions sent there.
Reference: Automatic email forwarding in Yahoo Mail