I have never diyed Polyspan, but regular silkspan and Plyspan (colored silksapn) many times.
Treat it like wallpaper. Put your water and dye in a wallpapering trough (looks like an upside down valve cover for a very large overhead valve motor). Roll up your paper, and let it sit in the water. The longer it is in, the deeper the color, of course.
When you take it out, hang it up just like you would your wash, on a line, and with clotheshangers. That will help even out things.
Then, choose whether you want to put it on pretty wet, or let it dry. If the latter, silkspan or Plyspan will shrink quite a bit.
Currell