It also similar to Facebook like button and twitter button.
You have to log in Google account to like this button.there have
different size of Button . add it as your like.
1. Log in to your blogger account and Go to Design >> Edit HTML
2. Put checked marked in Expand Widget Templates
3. Find this tag by using Ctrl+F </head>
4. Paste Below code before </head> tag
<script type="text/javascript" src="http://apis.google.com/js/plusone.js">
{lang: 'en-US'} </script>
5. If you want add this button Below your post title,or below post body.
6. Find this tag by using Ctrl+F <data:post.body>
7. Paste below code before the <data:post.body> tag for add button to below post title
Paste below code After the <data:post.body> tag for add button to below your post.
<div style='float:left'>There have different size button, If you want to change button size. Select Button below
<g:plusone size="standard" expr:href="data:post.url"/>
</div>
and Replace standard word (above code ) with your button size. you can see the button list below.
(small , medium , standard , tall )
8. Now you are done. Enjoy with New +1 Button