For a conversation :
Right click on the mail and then “tag conversation “and select the tag you want if already created or create a new one.
For a contact :
Right click on the contact then “tag contact” and select the tag you want if already created or create a new one.
For an event on the calendar :
Right click on the event then “tag appointment” and select the tag you want if already created or create a new one.
For a task : Right click on the task then “tag task” and select the tag you want if already created or create a new one.
For a document :
Right click on the document then “tag file” and select the tag you want if already created or create a new one.