Note: Make sure you use either Chrome or Firefox during the installation process.
Create your email signature on MySignature.io and follow these simple steps to install it in Yahoo:
After creating your email signature, click Save signature button under the signature preview.
Or, if you have saved your signature, find your signature on the Dashboard and click Preview and install icon.
Select the Yahoo icon in the list of email clients, then click Copy to clipboard button.
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 (on Windows PC) or Command + V (on Mac)) and go back to your Inbox.
Compose a new message then click Send to see and test your new email signature.
Congratulations! Now you will see your new signature in every email you send with Yahoo.