You need to be logged into Yahoo Mail.
Click on the "Gear" icon then open your Settings and then go to Writing email.
Then look further down in that "Settings" box and you will see a link to Accounts, where you can edit your signature.
You will then need to drag your Privyseal below into the signature editor.
Then save your email signature.
You should now test to see if your Privyseal is properly installed by composing a new email and seeing if your Privyseal is pulled into your new email with the current date and time. If it is properly installed, your Privyseal should always reflect the current date and time, as your Privyseal's details are checked regularly each day.