To create a new tag, navigate to Documents > Settings > Tags and select Add.
You only need to specify a name for a tag. You can also write an optional description. Setting a tag to Hidden will prevent the tag from being selected for future documents.
To assign tags to a document navigate to the Manage screen for the document you would like to tag and select the Details tab.
All of the tags available in your account will appear just below the document link. Tick the tags you would like to assign to the document and select Save.
To alter which tags are applied, tick/untick the tags accordingly and select Save again.