A few months ago I was messing with cards and wanted to create that square formation, which was very common. Then, I accidentally twisted my hand and 2 cards "stood up" which inspired me the create the full flourish.
This tutorial is almost 20 minutes long, filmed from 2 angles at the same time, pauses at times to explain specific points about the move and then continues. I don't think I could have done this any more detailed than I already have.
Small warning: this move can be really frustrating at times, but I think it's worth to learn, so don't give up!