There are a number of different ways to apply document tags. 

1.  From within a document

As you work through the document set available to you, it may occur as you go to add certain documents to a tag. This can be done from within a document by selecting the tag icon near the top-right of the page:

This will open the Select Document Tags window where you can select the tag(s) to apply to the document.

Alternatively, if you have a shortcut key set for a particular tag, you can press the shortcut key to apply a tag. E.g. if you had attached a tag to Shortcut Key 1, when within a document you can add that document to the tag by pressing “1” on your keyboard.

2. From the Documents view

Right click on a document or folder of documents in the index and select Tag:

If there are numerous documents to tag, you can simply select all the documents you want and then tag them together. For example, if you wished to export the documents at B/2 and B/6 from the below screenshot, you can left-click on one of the documents’ title (this will highlight the entry light-purple, indicating it’s selected). Then, to add more documents to the selection, you can hold down the CTRL key on your keyboard whilst left-clicking on the others. You should then see highlights as per the below screenshot:

To add more documents to the selection, you can simply scroll to the appropriate document(s) and once again hold down CTRL and the left-click on the relevant documents. Be aware that left-clicking otherwise will de-select all the documents you have selected up to that point. 

It is also possible to select a range of documents within one folder. For example, if you wished to tag all the documents from B/2 to B/6, you can left-click on one of these two documents, then, holding Shift on your keyboard, left-click the other of these two documents. This will select all the documents from B/2 to B/6, inclusive. i.e. you should click on the documents first and last in the range of documents you want selected.

Once all the desired documents are selected, right-click on the most recently selected entry and click Tag

3.  From the Search section, 

You can tag results from a search by selecting the tag button under Tools:

4. From the Notes section

Documents that have been annotated can also be tagged via the Tools menu in the top-right of the Notes page: