To install your email signature in Yahoo Mail, follow these steps:
Make sure to use Chrome Browser (Safari or Firefox will mess up the formatting)
Click on the 'Copy Signature' button on the Install Signature popup to copy your signature.
In Yahoo Mail, click on the 'Settings' option on the top right and select 'More Settings'.
In settings, select 'Mailboxes' from the left list and click on your email address under 'Mailbox List'.
On the right, under 'Signature' option, turn-on the signature by clicking on the small toggle button in front of the signature.
Click inside the signature text box and Paste your email signature (ctrl+V or cmd+V for Mac).
Click the 'Save' button at the bottom.
That’s it. Compose a new email to test your signature.