You can just knit the pattern flat, using the circular needles. They're made for flat knitting too!
But if you're eager to do something circular, you could knit the front and back together as one circular tube, from the bottom, up to the armholes, and then seperate them and follow the directions as given, with two separate halves. I've done that before; it's a nice compromise. The only potential complication is gauge. Gauge is often different when knit in the round, than knit flat (because you may purl at different tension than you knit, and with circular, you don't incorporate that purl stitch). So you may have to adapt the pattern for your two gauges.
Have fun with your new set!