Place the bobbins of yarn to be plyed on a Lazy Kate or spool rack. The bobbins are either placed side by side of one above the other, and they must be free to rotate, the yarns coming from underneath as they wind.
Patricia Baines, Spinning Wheels, Spinners & Spinning (New York: Charles Scribner's Sons, 1977, p. 217)
Yarns should come from underneath. That might have saved me from some snarls as I plyed.
And to further increase my ignominy, I failed to load the bobbins with even amounts of singles despite using a digital scale and deducting the bobbin weights. The top bobbin is empty and the bottom two bobbins look like they have about half an ounce left apiece.
I spun some more singles recently but they look a bit thinner, and being an inexperienced spinner I don't know if I want to introduce one thinner single into a three ply mix.