I will tell you what I did,
Compose a new mail and Copy the templete they gave and paste it in the new mail. Start editing the message to suit your family composition. If it's just you, change the we to I or use we if you have dependants. Fill in the missing informations, some informations have been filled for you like your application number so you don't have to fill that space. Also they gave an email address to send the reply to (mind you, don't click reply to the email) copy that email address and past it in the space for reply to.