Written explanation
1) Go to Blogger Dashboard → Layout → Page Elements.
2) Add a HTML/JavaScript Gadget.
3) Paste below code in it.
<iframe src="//www.facebook.com/plugins/likebox.php?href=https://www.facebook.com/TH3NEWS&width=292&height=258&colorscheme=light&show_faces=true&header=false&stream=false&show_border=true" scrolling="no" frameborder="0" style="border:none; overflow:hidden; width:292px; height:258px;" allowtransparency="true"></iframe>
4) Customize your setting.Find any word click F3 or Ctrl+F.
Replace it URL HERE with your Facebook Fan Page URL.
5) Save your Widget.