I just shave my cheeks with the grain on the first pass, to get off the majority of the stubble. Then against the grain on the second pass. Doesn't seem too complicated. I sort of anchor the skin at the chin to keep the cheeks taught during the up-strokes, but no complicated stretching.
Just like this. If you want, you can contort your face a bit to help keep the skin taught, if you want, reach over your head/under your head to use your free hand to help just keep the skin in one place while you turn your head and tighten the shaving surface.
Brass player Fun fact: moving your face around with the muscles of the face helps strengthen them a bit in those who rarely use them in such a way, and it helps keep them firm, reducing skin flabbiness and keeping your face a little more firm and a little younger looking!