After installation, click on the extension icon and select a setting. You will find yourself in the section where you can create a key, and if you already have a pair of keys, import them.
After completing the configuration, return to WebMail and click again on the extension icon. Select "Additionally", and then - "Activate in the current tab».
After completing the above steps, WebMail will display a new icon in the message composing window. By clicking on it, you can type the text of the letter, indicate which key owners can decrypt it, and encrypt the typed text. It will be added encrypted to the WebMail window.
The destination key will be automatically imported if it was previously uploaded by the owner to key server... Otherwise, you must first manually import the interlocutor's key. Likewise, your (public) key must be known to the addressee. Unlike a public key, a private key must be stored in a safe place and not transferred to anyone.
The official documentation for the extension is provided here.