There are some high quality free files if you are interested in those.
http://www.birdscards.com/doily-gatefold/ (she has several varieties)
And Monica has beautiful gatefold cards - some doily
http://monicascreativeroom.se/category/ ... old-cards/You can google and find others. There are pages of images on Pinterest and even google images. I would think , this is oversimplifying it, that you basically make a square, have a doily the same width, cut it in half and weld it (union in SCAL) to either side. As I said, that was a super simple explanation. It would probably be quite easy to understand if you cut one of Bird's files that cut beautifully and are a little simpler to cut than Monica's, and see how that goes together.
Hope this helps.