How do I attach tags to my documents? and how do I de-attach tags from my documents?

How do I attach tags to my documents? and how do I de-attach tags from my documents?

I recently started to use Zoho Writer and i am very enthousiastic. Using one or multiple tags for organising documents is a very good idea.
 
But I can not figure out how I can do that, in the help text I can not find very specific instructions.
 
How can I do this?