Note: Make sure you are using either Google Chrome or Firefox during the installation process.
Create your email signature on NEWOLDSTAMP.com and follow these simple steps to install it in Yahoo.
After you finish creating your email signature, click Save and Install button under the signature preview.
Or if you have saved your signature priorly, hover over your signature on the Dashboard and click Use button.
Select the option Other in the list of email clients.
Scroll down the Signature Preview and Click the Copy and Proceed button, which will save the signature code in the clipboard.
Open Yahoo and go to Settings → More Settings.
Choose Writing Email in the left menu and turn the signature ON.
Paste the signature into the edit box (Ctrl + V or Command + V) and go back to your Inbox.
Compose a new message to see and test your new email signature.
Congratulations! Now you will see your new signature in every email you send with Yahoo.