I dont know why but most of the people still don't know about the structured meta data & the awesomeness it can add to your site's listing in SERP ! Adding structured data to your site has been proven to be a successful way to increase click though rate, however for the first time when I saw a star rating in SERP for a specific query then the very first thing that hit my mind that the site must be a really good one for which it had been rated with 5 stars! This is exactly how a common user thinks, they dont know that this can be manipulated as well. Not only star ratings though...structured data can also be used to show navigation bread crumb links below the original site listing, for a long time Google had accepted three types of structured data to include in their search result & those are 1. Micro data 2. Microformats 3.RDFa . However to avoid the contradiction between datas , Google now currently advices to use microdata only. Lets get into the part for which you have been waiting so far..adding stars to your listing..yay!
First make sure what type of rating you want to use for your site, for example is it going to be aggregated review or a individual one? If it's about a aggregated review then the html should look somewhat like this...
<div itemscope itemtype="http://schema.org/Product"> <span style="display:none; padding:2px 0 0 10px;" itemprop="name">Website Design</span>
<div itemprop="aggregateRating" itemscope itemtype="http://schema.org/AggregateRating">
<meta content="1" itemprop="worstRating">
<meta content="5" itemprop="bestRating">
<p style=" font-size:11px; font-family:Arial, Helvetica, sans-serif; font-weight:bold; color:#fbcf3a; margin-left:15px;"><span itemprop="ratingValue">4.5</span> STARS OUT OF <span itemprop="reviewCount">7</span> REVIEWS</p>
If it's about s individual product review then it should be like this..
<div itemscope itemtype="http://data-vocabulary.org/Review">
<span itemprop="itemreviewed">White shirt</span>
Reviewed by <span itemprop="reviewer">Adrian thomas</span> on
<time itemprop="dtreviewed" datetime="2009-01-06">Jan 6</time>.
<span itemprop="summary">A great one for a night out</span>
<span itemprop="description">A great shirt for all purpose use, flaunt your style</span>
Rating: <span itemprop="rating">4.5</span>
Now once you are done with the adding part, dont forget to check out if you have done it perfectly, use Google structure data testing tool & see the search result preview, are the stars there? Awesome isn't it?