When you first launch the launcher you will see this, the login screen and the log. You will need to login the same way as you would the official launcher, use your Minecraft username (e-mail address if account is migrated) and password. You will need a valid premium Minecraft account in order to proceed. *Note: Your password is never stored and is sent to only Mojang's authentication servers once (assuming you login once). If you check the Remember Me box you session information will be saved and your UUID and Client Token will be sent back to the auth server to refresh your session. You can browse the code to see for yourself. Once logged in you will continue to the launcher screen, click here for information about the launcher screen.