Over at XD they have a short walk through to make some standard icons for your stacks.
There was a termainal hack that let you do this but someone went ahead and took care of it for you. All you need to do is download the icons and then drag them into the folder you want them to show on the to of the stack. Example just drag the Downloads icon to the downloads folder on the doc and it will show the icon.
The only thing you need to do is right click on the stack and make sure that you are sorting by date modified.