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
Select All and Copy the code
Click Edit this content
Click + Add content
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!