Now they have announced that this tool has been extended to cover schema.org definitions of seven new types of things:
- Local businesses
- Software applications
- TV episodes
The basic instructions for using the DH tool have not changed: visit Webmaster Tools, select your site, click the "Optimization" link in the left sidebar, and click "Data Highlighter".
Google now advise that "The tagging process takes about 5 minutes for a single page, or about 15 minutes for a pattern of consistently formatted pages."
After you have done some tagging, you can verify Google's understanding of your structured data. If Google has understood your work correctly, and you "publish" it to Google, then, when your site is displayed in search results, Google will use enhanced displays of information "like prices, reviews, and ratings".
It will take time for this to happen: Google needs to re-index your blog-posts before it takes effect.
Note: schema.org is a markup language that this tool is based on. However not all item types are covered by the tool at the moment. So, for example, we can only use the Data Highlighter tool to show Google how articles, products, TV-show episodes etc are represented on our blogs. It doesn't go into more detail for things like blog-posts which schema.org does have definitions for.