Last time i was searching for the same. But didn't do any update or written post regarding this. As this is known to all the bloggers. But for the freshers who have seen WordPress widget like to add the same thing in blogger posts also.
This experiment i did to my blog post. Nothing was working. Today i am posting this because some one might required or wanted to add /learn how to do the changes. 
Here the below post which gives you the information http://www.poketors.com/2012/02/how-to-add-thumbs-up-rating-widget-to.html. Here they have explained exactly what has to be done? Lets try to learn this. from the link given. Clean and neat explain has been given. You will directly understand what has to be done? So if you like this give a credit by thumbs up or comment it will help them to create more like this.