Run the email client you installed (Thunderbird or SeaMonkey). You will notice a new submenu called OpenPGP in the menu bar: that's the submenu for all Enigmail operations.
Select OpenPGP → Preferences to have access to the Enigmail preferences window. There is a toggle button named Display Expert Settings / Hide Expert Settings that displays or hides several options to fine tune your installation of Enigmail. Throughout the rest of this handbook we'll assume you stay in Expert Settings mode.
Enigmail is equipped with a Setup Wizard that can guide you through all steps to configure Enigmail and have it ready to use. This is intended for inexperienced users, but is not necessary: you may as well configure Enigmail by hand, which will grant you a deeper knowledge of the mechanisms of Enigmail.
The rest of this chapter is a step-by-step guide to the Setup Wizard followed by a very basic explanation of the signing and encryption functions. If you decide not to use the Setup Wizard, you can go directly to the next chapter.