Get a verification key from Google Webmaster Tools. Copy it to the window of an open text editor. It should look something like this:
<meta name="google-site-verification" content="SOMESTRINGGOESHERE" />
The important part is the value of the "content" attribute, so remove the rest of the string so it looks like the ofllowing:
Make sure the Metatag: Verification module is enabled, it comes with the main Metatag module.