Probably this is going to be one of my shortest blog post:Select a document which need to be tagged (add tick mark).On the top ribbon, you can find "Tags and Notes" . Click on that. Add the necessary tag.The tags gets mapped to the document.
After crawler runs, this tags will be visible inside the OOB Tag Cloud Web Part present under the same url. Cheers, Avik Technorati Tags: Tag a Document,Tag Cloud,SP 2010