You can do them on one circ at the same time, you don't have to, or you can knit in the round up to the armholes. Many patterns call for knitting on straight needles, but a lot of knitters don't use them, so knit back and forth on circs. The needle sizes are 3 and 6, you do the ribbing with the small ones, then change to the larger ones for the body after 3 inches or so. I think the confusion comes with the layout of the pattern with the stitch pattern written out first, then the sweater pattern itself. You can copy and paste each section into a separate file, then print them out. Refer to the sweater pattern for getting started and the shaping, then when you get into the body of the sweater, follow the stitch pattern.
sue- knitting heretic