After I flipped through David Coffin's Shirtmaking book, I ordered myself a flat-felling foot from my local sewing store. It came in, I went to pick it up, and I asked the relevant question - How do you use it?
We couldn't figure it out, and so the guy turned to an old Singer reference book - dated 1946. (I didn't take a picture of the cover, sorry!)
So, here's how, in Singer's words:
Enjoy! As for me, now that I know, I'll be practising.