Navigate to https://developers.facebook.com/docs/plugins/follow-button. From there, finish the URL of your Facebook profile. Then select the button size and any other custom elements you wish.

When you're ready, click Get Code

Click IFrame

Select All and Copy the code

Click Edit this content

Click + Add content

Click HTML

Paste the HTML code in and click Save

Click Save and close

There's your button down at the bottom. That's it! You're done!