Install the package.
Sharing the public key
To distribute the public key one may upload it to a keyserver.
Encryption does not always work properly with emails containing HTML. It is best to use plain text by choosing Options > Delivery Format > Plain Text Only in the new email window.
Once the email is finished it can be signed through the OpenPGP menu.
Assuming that the email was encrypted properly, just trying to open it should result in a pop-up asking to type in the keyphrase.