If you have a mobile-friendly site that we detect as one, your site is very likely to get a "mobile-friendly" label in our search results. This will help searchers know you're more likely to treat them nicely because you've optimized your site for their small screens.
So how do you go about doing this? It's very easy, conceptually:
(1) have a mobile-friendly site
(2) make sure Google indexing has access to the CSS, JS, and images so that when we render the page we can detect it's mobile-friendly.
Simple, right? :)
But, no, really, how do you do it? Depends on how your site currently works, and we have a ton of new guidance to which many of us contributed to help you. We have guidance whether you're using a CMS (Wordpress, Drupal, many others) or have a custom site.
The biggest source of wrong classification (i.e. how you can be hurting your site's indexing) is if you block Googlebot from crawling the CSS, JS and images on the mobile site, or if you block Googlebot from mobile site completely. These are easy fixes.
Dig in for more:
The Mobile-friendly test lets you see if a particular page is mobile-friendly: http://goo.gl/CYEBZK
The Mobile Usability Report highlights mobile issues that we found across a site: http://goo.gl/gIvTVj
Our Developer Site has brand-new documentation on how to create a mobile-friendly site, including how-to guides for common platforms: http://goo.gl/PqVm5F
And the announcement from yesterday: http://goo.gl/6tLfvo