To add an unsubscribe link to the footer of your outgoing emails, use the tool provided by your newsletter software. In some cases, this is as simple as clicking a button and typing a few words. Sometimes, you will need to write some simple code using HTML brackets.
It's extremely important to add an unsubscribe link to your newsletters because it allows the consumer to opt out should they feel overwhelmed by the amount of emails they're receiving. It's also very important to make your unsubscribe link a plain text link rather than an image-based one as many users' email clients automatically disable images from most senders.