Here is the search results I found:
Gallery at Splitcoaststampers
Not all of them are bags, but in scrolling down there are lots of good samples.
Looks like you measure and cut the cardstock to the size of the display top you want. - Some people have a large piece to decorate, some have a smaller piece. Then fold over the bag top and staple the cardstock in place (which seals the bag closed).
Then decorate it the way you want with DP, stamped images, stickers, whatever! Looks fun. I will post in my gallery when I finish my Valentine baggies.
Good luck!