1. Go to the Yahoo Mail settings menu and select more settings.
2. Now under the mailbox list, you need to select the Yahoo email account that you want to forward.
3. Enter your Gmail account email address in the forward section where you want to forward your Yahoo email.
4. Now select Verify
5. Now open your Gmail account and check the inbox for Yahoo verification email. Follow the instructions in the email to verify that you want to forward yahoo mail to gmail.
After following the above steps the emails from your Yahoo account will be forwarded to your Gmail account until you change the settings once again.