Select the check box to choose multiple documents that you want to tag and hover over it, then select > Add Tags. (Optional) For the selected documents, right-click and select Add Tags.
The Add Tags for Selected Documents screen is displayed with Tags to Assign field.

Select Add. 
The Enter tag text box is displayed.

Enter a name for the new tag you are creating and select Update. 
The new tag is displayed under Tags to Assign. 

Select Add Tags. 
The Adding 1 Tag to... screen is displayed and the tag is assigned to all the selected documents.

Select Close.