How to add tags to files in Windows 7 so that they appear in search results
You do it from windows explorer view
Just select a file that can have tags added to it and adjust those in the bottom pane. Doesn't matter which view you're using. The link I provided in my comment should help as well. You need to identify that the file type will allow tags as some will not or require third party tools - this link provides a solution for .pdf files, for instance