The format is straightforward. All you need is a plain text file with one URL per line. An excerpt of a valid file might look like the following:
# Contacted owner of spamdomain1.com on 7/1/2012 to
# ask for link removal but got no response
domain:spamdomain1.com
# Owner of spamdomain2.com removed most links, but missed these
http://www.spamdomain2.com/contentA.html
http://www.spamdomain2.com/contentB.html
http://www.spamdomain2.com/contentC.html
In this example, lines that begin with a pound sign (#) are considered comments and Google ignores them. The “domain:” keyword indicates that you’d like to disavow links from all pages on a particular site (in this case, “spamdomain1.com”). You can also request to disavow links on specific pages (in this case, three individual pages on spamdomain2.com). We currently support one disavowal file per site and the file is shared among site owners in Webmaster Tools. If you want to update the file, you’ll need to download the existing file, modify it, and upload the new one. The file size limit is 2MB.
One great place to start looking for bad links is the “Links to Your Site” feature in Webmaster Tools. From the homepage, select the site you want, navigate to Traffic > Links to Your Site > Who links the most > More, then click one of the download buttons. This file lists pages that link to your site. If you click “Download latest links,” you’ll see dates as well. This can be a great place to start your investigation, but be sure you don’t upload the entire list of links to your site -- you don’t want to disavow all your links!
To learn more about the feature, check out our Help Center, and we’d welcome your comments and questions in our forum. You’ll also find a video about the tool and a quick Q&A below.