It is very easy to add google +1 button to your site. Follow the following steps:

1. Go to the site  (http://www.google.com/webmasters/+1/button/index.html)
2. Select the size and type of button (Annotation) you need. 
3. Go to advance option
4. Select Asynchronous and put the link of your site
5. Copy and past < g:plusone > </ g:plusone >
   tag where you want the button to be displayed
6. Place the < script >
   < /script >
   tag anywhere in the page (best practice is to put it inside the head tag)
   
7. Update the meta tag as per your need which enable fast indexing of your page in the search engine.