Simply add the 'Tags' that you require for each page or file. Tags that you have already created will appear as you start typing. 

You can also manage tags in bulk here in the 'Tags' section.

PRO TIP: 

It is always best to be as granular as possible when tagging. For example:

Blue Jeans should be two tags, 'Blue' and 'Jeans' not 'Blue Jeans'

This makes for flexible widget management, both now and in the future. For example, your widget might show Blue Jeans today, however, in the future, you might want it to show all blue clothing or jeans of all colours. Therefore, it might also be an idea to apply tags for the categories that the page falls within, for example:

'Blue', 'Jeans', 'Colours', 'Clothing'

If you remain granular it makes it easy to swap and change tag combinations on the fly.