Just install the package from the package repository or download it from our.umbraco.org. Please do NOT rename the installed data types, otherwise they will not work.
In your content nodes, you will now get a list of checkboxes for all your Css- or Js-Files lying directly in the css or script folder. You can choose the files you want and publish the side. If you create a child node which contains the picker too under your current node, the files from all parent nodes will be checked automatically for you. If you don’t want to include one of these files just uncheck it and save and it’s gone.
If you found bugs or have questions visit the forum for the package on our.umbraco.org