open prontoeditNG, open up a page of any particular device, click on one of the existing buttons, a box appears entitled "button properties" click on "appearance" remove both versions of the existing button, then click on "insert", browse to where you have stored the downloaded button set and double click the relevant button into place.
once you have a button set up, you can export it to the gallery for use in other pages, simply click "tools" then "gallery" then "buttons" then "insert", choose your new button in the dropdown and away you go.....
bloody complicated i know....