1. In your WordPress Dashboard find the menu on your left.
2. Select "Facebook Button" => "Settings".
3. Find "Buttons" option and mark the necessary checkboxes near the buttons you would like to add to your post or page. Enter your Facebook ID or Username to allow users to follow you on Facebook next to "Facebook ID or Username" checkbox in "Profile URL Button" section.
4. Then click "Save Changes" button. In order to get more detailed information, check the Facebook Button - How to Use Instruction.