For manual adding see here http://mashable.com/2011/06/01/plus-1-button-wordpress/
Simply way http://itsmyownplanet.blogspot.com/2011/06/add-google-1-button-to-your-website.html
Plugin is here
For more info you can join to facebook page Google +1 Button
Nice article.but i already written this article with much simplicity.
[How to] Add Google +1 Button to your website – blog [Ready] (Google Webmaster Central)